Have a look at Neverland, the classic story reworked as a sandbox island hexcrawl. It's a very accessibly priced hardback, and a beauty to boot. It markets itself as 5e-compatible but is very much in the vein of OSR works like Dolmenwood and Hot Springs Island - proper thought has been given to how everything reads and sits together for reference.
Really nice dice and dice tray. I would also recommend some killer books, like Neverland. Lastly, agree to play in a one-shot of his. He'd love for you to experience it!
Neverland
The author is also a children's illustrator by trade. It also has short stories set in the darker world of Peter Pan to set the mood: https://www.amazon.co.uk/Neverland-Fantasy-Role-Playing-Andrew-Kolb/dp/1524860204/ref=sr\_1\_1?crid=1T7MEQ9MQHMKT&keywords=neverland&qid=1644962596&sprefix=neverland%2Caps%2C96&sr=8-1
We're fighting uphill against unclear jargon in our hobby, so I feel for you. And the customer, but that's secondary.
If you look at the advertising for mainstream system-neutral products doing what yours does, it may give you the accepted formula. This example is one I bought.
I know that's a setting and not a module in particular, but you can discern how well it connects to what you make.
Check out the Neverland book! Super affordable, crazy beautiful and full of awesome ideas.. factions, ready made dungeons, a whole bestiary. Plus, it's made first with 5E in mind.
DOLMENWOOD from Necrotic Gnome who was the creator of the retroclone Old-School Essentials. Its very much a fairytale campaign illustrated in the vein of a storybook.
Necrotic Gnome has also put out Winter's Daughter, Hold in the Oak, and Incandescent Grottos. All of which match what you are looking for.
NEVERLAND was written for 5e but lists several OSR sources as inspiration. Its a hexcrawl in the vein of The Dark of Hot Springs Island in Never Never Lands of Peter Pan fame. Also looks very much like a children's book. Its really amazing.
THE BLACKWYRM of BRANDENSFORD is another very nice fairy-tale like setting including some pretty cute interactions with village folks and their lives.
And while a little darker, IN THE SHADOW of TOWER SILVERAXE is quite good as well and fits in the same "vanilla D&D" vein that you might be looking for.
Not sure how micro you want, but Neverland by Andrew Kolb.
You could also look at my procedural In the Heart of Oz setting.
https://www.amazon.com/Neverland-Fantasy-Role-Playing-Andrew-Kolb/dp/1524860204
If you would like to see what's in it, Ben at Questing Beast did a flip-through.
It's so weird to see traditional publishers like Simon & Schuster getting into ttrpgs. It me reminds me of this neverland 5e setting book.
https://www.amazon.ca/Neverland-Fantasy-Role-Playing-Andrew-Kolb/dp/1524860204
Nope. this one.
I've been playing with my son since he was 4 years old (he's 6 years old now) usingthese rules-lite house rules that are OSR compatible.
All of the Necrotic Gnome adventures (except Blood King) are pretty chill violence-wise or can be. Winter's Daughter stands out as chill for kids.
I've also had a good time with Neverland, Blackapple Brugh, The Black Wyrm of Brandonsford (especially this one),Hideous Daylight, and Palace on the Pink Waves. None of them are necessarily super violent if you just tone it down appropriately. My son and I have played through all of these and had a great time!
Good luck playing with your kids. It's the super best!
I have yet to run it, but I am extremely impressed by the Neverland adventure setting book: https://www.amazon.com/Neverland-Fantasy-Role-Playing-Andrew-Kolb/dp/1524860204
The production of the book is unusually high quality - above and beyond the official WotC books and other 3rd party books I own. The organization and layout is also very good. It includes a large font easy-to-read table of contents at the beginning to help the DM find what they want. It even includes a bound bookmark which is something I've never seen in any other D&D books. It's primarily a hex-crawl island sandbox adventure with several points of interest to explore. Even if you don't run the adventure in its entirety, it contains dozens of custom creatures, npcs and maps that could be dropped into other adventures.
Alright since I did read that he would be the DM, I am here to suggest some things that might be a bit off the beaten path.
Most people have mentioned already that a subscription to DND beyond is very helpful since it essentially provides an affordable online equivalent to all the basic books and means he can run the game from the laptop. (Master subscription probably the better choice since he'd be running a game)
Now for things that may be a little different.
Third Party Adventures. (These are books of adventures to run with 5th edition dungeons and dragons but not published by wizards of the coast)
Escape from Skullcano Island *
An adventure that takes place on an inhospitable island full of giant monsters. Perfect if he happens to be a fan of B-Monster movies and giant monster pics like Godzilla. Make sure to choose the print version and then email him the PDF because they'll send both.
one of the better DM notebooks I have seen. Comes in 4 colors and full of handy tables and graphs. Perfect if he wants to write his own setting.
Glimmering Crypt of the Ioun King *
Another Weird adventure from the same makers of Escape from Skullcano Island. Filled with Slime Monsters and Crystals. Make sure to choose the print version and then email him the PDF because they'll send both.
Designed to be used with any game rules (meaning it has no specific stats so this assumes that whoever is using it has all the stuff to run a game... in this case the a subscription to DND beyond should give him everything he needs.) A weird setting on three mountains filled with mystery and danger. The artwork is something special. an especially weird setting
If he is a fan of disney this would be a very interesting setting to run. Set in Neverland you can encounter the Darling children in Neverland... though things are a little different from the Disney movie. it is filled with dark and sinister elements. A particularly fancy book too.
I would say he'd only need one of the above books if he has the three basic DND books (Players Handbook, Dungeon Master's guide, Monster Manual). Just some suggestions in case you wanted to give him that is related to dungeons and dragons but that he might not come across himself.
*if you order these by today (December 8th) you should get them in time for christmas with regular shipping
Hands down, Neverland.
Even if I didn't play rpg's i would want it on my shelf
https://www.amazon.com/dp/1524860204/ref=cm_sw_r_cp_apa_glt_fabc_H4XK255VZXR238Q2SC52
Peter Pan would run well in Dungeon World or even Fellowship (https://www.drivethrurpg.com/product/177662/Fellowship-2nd-Edition--A-Tabletop-Adventure-Game).
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If you like Peter Pan, then the setting book Neverland will be your jam.
It is probably in the top 10 of all RPG books, ever. It is amazing.
This'll be a bit ramble-y, so I apologize in advance.
I think the most useful setting books are less generic, ruleless encyclopedias and are more akin to mini sandboxes/toolkits. I'll use both the Neverland setting book and Eberron: Rising from the Last War as templates and examples.
A setting encyclopedia only helps us insofar as it provides us the lore that can fuel a game, but very little else. Meanwhile, every good RPG product should have direct utility in your game, whether it's by providing rules, frameworks, character options, monsters, encounters, or even just advice. So a setting guide must provide both crucial lore and gameable materials.
Looking at Eberron: Rising from the Last War's character options, we can see it immediately starts providing gameable materials. It gives players new races and a new class appropriate for the setting, and also gives the option for a group patron system and Dragonmarked PCs, which play into the noir intrigue that Eberron is fueled by. It takes base 5e, and tailors the core group dynamic to fit its world. Neverland, sadly, doesn't provide much help in that regard, but does list sample PCs that could be used as inspiration.
Moving into rules and frameworks, we can see Neverland really shine. The whole concept of the setting is of an island hexcrawl, and procedures for navigating, exploring, and interacting with the island are here. In addition, the island has various schedules (weird events that happen at 6am and 6pm and the island reacting to the phases of the moon). Neverland also provides plenty of "downtime" activities that the PCs could interact with that are tactile and expressive of the world in the form of Fairs, Pirate Parties, and Mermaid Concerts. You can probably find similar things in Eberron when it discusses Khovire or Sharn, but Neverland places these activities front-and-center.
Moving on to resources, both have a large cast of characters and factions that the players can meaningfully interact with (that is core to the settings' themes), and offer tools to generate monsters, treasure, and adventures (or offer pre-made adventuring sites) that follow their themes and goals.
In conclusion, the utility both these books provide illustrate what can make a good setting book. They need to not just offer lore, but concrete, gameable elements that help me run adventures in those worlds. I don't really care whether or not they're system-agnostic, only that they help me run the game I want to run in that setting.