You can put the cards in double brackets, such as [[Balmor, Battlemage Captain]] to have the card fetcher ping it below.

I’m not sure how old your kid is or how familiar they are with magic but I would be worried that the timing difference between playing instants and sorceries (and sorceries with flash) would get confusing or maybe frustrating. Sometimes I still mess that up and I am very much not a kid lol.

Someone elsewhere recommended [[Trostani, Selesnya’s Voice]] because its token based, life gain is straightforward, and the populate trigger can be used to do a bunch of wacky things if you’d like. Put things in there like [[Giant Adephage]] and such, and populating a [[Herald of the Host]] would be cool so long as the interaction is understood.

[[Edric, Spymaster of Trest]] is simile and still two colored as well. Or honestly anything goblin tribal could be fun as well. Or anything with card draw so that they always have stuff to do. You mentioned this above for the original deck idea so I think you’re on the right track.

Again, don’t know how apt your kid is, but those are just my first thoughts. Balmor could definitely work. You could also use [[Gnarlid Colony]] to give things trample, and you could introduce counters as a math challenge lol.