Lucky for me I've never had this problem!
The younest person I've had hold Dewey was 11 (I'm only 14 myself :P) and she has a hamster of her own as well and she's one of my best friends as we've known eachother since we were toddlers. But Dewey is a pretty calm hamster and doesn't mind much.
I don't have any cousins or neices or nephews or anything. But I did have a 5 year old at my house once and I just moved Dewey and Bloo's cages to a different room so she wouldn't see them
But I would keep the hams away form the little kids like you said if they just can't figure out how to handle them correctly. Maybe just do as someone else said and just hold him while they hand him treats??