I am guardian to my neice and nephew currently, but they are older than your children I think.

Are they old enough for finger painting? Thats always fun.

play dough is great too!

I do a lot of beading, but that is more of an older kid thing. When they get older it helps a lot with fine motor skills.

Another thing may be to get them one of those little indoor trampolines. That way they can exercise inside. They make some cool indoor slides and things for young kids too.

When my neice was young she loved the fisher price kitchen things. She would make plastic food for everyone.

Maybe do some cooking with them too. Making cookies, etc. is always fun.

Sorry I'm not much help. I only have experience with older children. I don't have any of my own.