One of the best Pokemon games to date.
I do have a bit of a bias toward this game, because it was my first venture into the world of Pokemon, and Nintendo made games.
I first played this game when I was 6, and I picked up on the concept quite easily. This game blew my little 6 year old mind away. As far as GBA graphics go, they were very nice. Pokemon has a pick up and play nature, perfect for a handheld device.
This game is a remake (or revamp) of Pokemon red and blue, the original games in the series. This game starts at the very beginning in Pallet Town, with all the original 151 pokemon catchable (well, except Mew). As a 6 year old to now I've logged about 70 hours into the game, but it took me nearly 30 to beat the main game because I wanted to see every nook and cranny of Kanto. Playing the game now, it takes me about 6 hours to beat.
So, in short, if your child has a ds with a gba slot, get them this game.