I don't have kids or little sisters or brothers (well I do, but the youngest is 18!) so I don't really know. I had kittens since I was 7 or 8, and I was old enough to pick them up and care for them completely on my own, even newborn ones that I handreared. But I think I was old enough to not drop them, and probably the most careful person in the world with them! So I'd say it depends on the kid, I wouldn't let any 7 year old pick up my pets, unless I knew how careful they were, but I did it myself at that age!
Recently my sister's friend was over with her three year old, and I let him see all my pets. He was very gentle with my puppy and kittens, and just rubbed them gently. I let him see the guinea pigs and rabbits but not pick them up, for their own safety, and just in case one of them nipped or scratched him too! I wouldn't have minded him picking up the kittens, as they wouldn't have been hurt if he dropped them (they're 2 months old) but he was very good and didn't try to pick them up.
I think if you just explain to them that the guinea pigs are very fragile and can run out of your hand (as they sometimes try to do!) then they'll understand, and they can still cuddle them on their laps. I think it's a great idea to get them to help out with some chores, feeding, giving hay, bathing etc. so they learn how to take care of an animal, but without having to do all the hard work!