Purely anecdotal I know, but I grew up with cats (the first one was there before me) and they remained unharmed!
If you supervise the kids with the kittens and teach them how to respect them, I don't see why there should be a problem. The 6yo should be plenty old enough anyway, it's the 3.5yo you'll have to watch of course.
I've taught DS to be gentle with animals from a young age (he's 4 now). I found it helped to give him something positive to do, rather than just telling him what not to do.
With cats it's all about trust. I've taught DS that cats get scared very easily, so if he wants a cat to be his friend, he needs to crouch down, to make himself small, as they think you're bit scary if you're big. Also to stay still and hold your hand out (fist closed, back of hand) and let the cat come and smell you. I've explained that cats really like to smell you, and you need to let them smell you before you can stroke them.
We made it a game, and he understood fine it at 3.5.
The bit above is for adults cats who don't know us (e.g. at friends or in the street), but you get the idea! Show your little one what to do, in a game, and supervise and it'll be just fine I'm sure.
Enjoy your kittens, I'm jealous!