If you'd rather not build it yourself, you could get one of these.
DAW is definitely a little over you and your 6 year olds head but basically it's a program specifically used for recording music. They're expensive they're computer intensive. You can get free ones such as "Reaper" but there is still going it be a lot you need to learn.
You can use a midi box to get sounds out of the keyboard like this to avoid using a computer:
https://www.amazon.com/dp/B00VHKM190/ref=cm_sw_r_cp_apa_glt_fabc_PH290AKNXQCWZFF3DD5T
I'd recommend looking on craigslist or Facebook market place for an actual digital keyboard that will have sounds programmed in and will be easier to learn. If your 6 year old takes on music, they will figure out how to use the midi eventually! You can get a quality used Yamaha for around $100 and they have great sounds and functionality. The last thing I'd want is the kid to become discouraged due to a difficult experience of turning on the piano. If money is a concern you could almost certainly find one for free or close to it, but it may need replaced before long.
These are fine enough for practice. USB host input so you can plug the keyboard straight in, built in battery, but no speaker, only headphone out
https://www.amazon.com/midiplus-miniEngine-USB-Sound-Module/dp/B00VHKM190
MIDI player that runs on batteries.
Expensive, but the sounds are pretty awesome. MIDIPlus makes several sound modules around $100. This one is the cheapest, at $89 on Amazon. Sounds pretty good in this YouTube video. There are a couple EP sounds starting around 4:13.
Well, if you need sounds - just look for SoundFonts... But, there will be a learning curve. The easiest thing is just to buy a device with sounds....
I have one of these. I bought it just for that. Something to plug into my keyboard. https://www.amazon.com/midiplus-miniEngine-USB-Sound-Module/dp/B00VHKM190
If you want just basic sounds then the midiplus mini engine is quite handy.
https://www.amazon.com/midiplus-miniEngine-USB-Sound-Module/dp/B00VHKM190
> https://www.amazon.com/midiplus-miniEngine-USB-Sound-Module/dp/B00VHKM190/
This seems to be what I want. Just slightly pricier than I'd hoped ><; Thanks for the tip.
I'm going to pick up one of these as it claims to work as a MIDI host (I should be able to get it cheaper where I live) - if it works it will be a good budget option.