Ooh, how fun!
In terms of stranded colourwork, Tin Can Knits have lots of children’s patterns with colourful designs. There are also tutorials on designing your own colour scheme or stranded pattern using their sweater designs as a canvas.
https://tincanknits.com/
You could also try a stranded colourwork design from an adult sized down. It’s not too difficult with a swatch and some basic calculations.
Drops have lots of free colourwork designs that could be adapted this way. (You can take the stitch pattern and put it on a plainer style of sweater too.) You can browse through their designs on Ravelry. A lot of these don’t have the “all over” colourwork that you are looking for, but that is fairly easy to change.
https://www.ravelry.com/patterns/search#pc=sweater&pa=stranded&designer-link=drops-design&sort=popularity&view=captioned_thumbs
Or it might be fun to design your own motifs together using graph paper or drawing from a sourcebook such as “200 Fair Isle Motifs” by Mary Jane Mucklestone.
Even a cowl could be inspiration: https://www.ravelry.com/patterns/library/28-mobius-cowl
I don’t know of many intarsia patterns for children that are in print. You can often find 80s books or pamphlets at the op shop with intarsia children’s designs because that is the last time intarsia was “in fashion”. Also your local library may have older Rowan books with Kaffe Fassett designs.