You could always get him this book, I know its hugely recommended across the astrophotography groups. https://www.amazon.com/100-Best-Astrophotography-Targets-Telescopes/dp/1441906029/ref=sr_1_1?dchild=1&keywords=The+100+Best+Astrophotography+Targets%3A+A+Monthly+Guide+for&qid=1610581680&s=books&sr=1-1 and then maybe add a giftcard on top of it
You can watch PBS Space Time with her for free. The episodes are short. Unless she's an unusually precocious 5 year old, she will not understand much, but you'll learn things that will help you answer her questions, and she'll enjoy the videos and tone and especially that she's watching it with you.
She might also enjoy things like videos that show the relative sizes of planets and different types of stars.
Same goes for Cosmos episodes: let her take what she can from it now; she can come back to them again when she's ready to get more of them. Likewise books - I started buying the Dorling Kindersley books for my nephews when they were 5. Mostly they liked the illustrations and the names, but the books are still with them and now they're reading the text too (they're more about the dinosaurs but the same applies).
Another thing she might enjoy is glow in the dark ceiling star maps. Having real constellations on her bedroom ceiling will be very exciting!
Does anyone have experience or own this book?
I just picked it up on amazon, and despite being advertised as having full color images (which would be very helpful considering the subject material), all the images are in black and white and look like they were simply photocopied (very poor quality). I sent it back to amazon and I'm waiting for a new copy, but I'm curious if I'm missing something...
Doing it optically is a fun project, but worth pointing out that the various rotations are so well known and studied that once you calibrate for position, a purely mechanical positioning system is more than adequate, and computer control has been a solved problem for decades. You might want to read Trueblood and Genet to get a feel for the control and automation side, and start with a mathematical/physical solution - you can add vision later.
Edit: specifically https://www.amazon.com/Microcomputer-Control-Telescopes-Mark-Trueblood/dp/0943396050 - the hardware is much easier these days, but the control theory and algorithms are the same.