The 2DS is also backwards compatible with the original DS, so you've got a huuuuge library to choose from. Your kid is going to have lots of fun 🙂
For Pokemon, you are best getting the latest version- it is likely that their friends will have the same game, and want to trade Pokemon with them/battle them. So I would probably go for either Pokemon Ultra Sun or Pokemon Ultra Moon- but check with your kid/your kid's friends/friends' parents about which version their friends have. You don't want your kid to be left out because they have the "wrong" version!
Your kid might also like the Detective Pikachu game. This is a spin-off Pokemon game, not a "normal" Pokemon game, but it is meant to be quite fun- and it ties in with the film that just came out.
If you want other kid friendly games, there's some great platformer games out there:
New Super Mario Bros
New Super Mario Bros 2
Super Mario 3D Land
Donkey Kong Country Returns
Yoshi's Wooly World
And I would also highly recommend Legend of Zelda - A Link Between Worlds, that's another great kid friendly game.