For younger kids I think it is a super fun challenge to find ways to share advanced math ideas in ways that are accessible to them. Our paper folding "FamilyMath1" was already shared above, but here is FamilyMath2 - my younger son was 5 in these videos (and I just did FamilyMath985 with him today!):

Another fun thing I did with both kids when they were learning arithmetic was making binary adding machines out of duplo blocks - this is quite similar to the already mentioned "exploding dots" exercise from James Tanton:

It was a surprise to me how far you could go just with blocks -> here's a fraction division example:

And here's a "proof with blocks" that a negative times a negative is a positive:

For the 6th grader - by luck I'd previously written a post about 15 fun projects for a 6th grade math camp. I'd be happy to go into detail on any of these projects if there are any specific quesitons: