A 6 year old kid usually can't handle too many keys like we gamers do. And the content has to be kept in the control too. So, Logically, let me tell you all those games which I MYSELF PLAYED when I was 6 years old (only DOS was there back then):
1. Dave,
2. Mario (older DOS version),
3. Pac man, (easiest, only four directional keys are used to play it)
4. Hocus Pocus (looked amazing and colorful for its time)
5. Arkanoid, (some board game I think)
You can get these games from the internet instantly & easily, and in low file size. Moreover, these games are Abandonware, which means their creators have no commercial interest in them, or have declared them free. They're legal.
Ahh... those old days, when stuff used to be sold on 5 1/4th inch disks having 1.2 MB, and a 3.5 inch disk with 1.44 MB seemed so futuristic.