AndyT:Also I never got an answer. Can my grandson and I do a dinosaur diorama?
Well, dunno about "joint efforts", but if he feels that you won't slow him down or mess it up, then sure... 
On a more serious note.. How old is he? I'm assuming that he's fairly young, although older kids have interests in dinosaurs too, it's generally confined to the "12 & Under" age-group.. (Mine's 10, and he's a bit past that stage, although he was a fanatic about 'em only a couple short years ago)
Some dioramas are going to be military subjects and may very well be rated "R" simply because of the possiblity of graphic violence if a diorama contains combat scenes and/or the aftermath, which almost always involves casualties and blood being modeled...
I won't "limit" the violence level of the others' dioramas to "PG-13" (although they may freely do so themselves)... So as long as you are willing to explain the scenes to him (if he views them all) and that they're only models, then, fine... Just so ya know also, there is no "Junior Category"...