Me and my Son (3 1/2) have so far assembled 4 Kits together:
1.) Zoid Puma
2.) Halo Robot Car
3.) Zoid Shot Walker
4.) Zoid Road skipper
I cut and trim the parts for him and tell him how to join them, he is really starting to get into snapping things together. Granted I am still doing the majority of work.
He already knows that my Models are for adults and that the simpler stuff is his age-group. He also loves to watch me work on my Models.