[McGroin]

For Soldier S's sake:
Silouehette Mirage - Sidescroller shooter/beat-em up w/ kiddy egde
Einhander - Advanced Sidescroller shooter
Metal Slug series - sidescroller shooter (theres been a couple of releases for ps1)
Ridge Racer series
Ace Combat

I think most of these games except for Ridge Racer will have steep learning curves for a six year old, but all worth playing.