Okay, this is going to sound dumb, but I was at a sleepover when I was 7 or 8 and this memory has stuck with me 40 years later as the best time EVER. Go to the dollar store and buy like, a case of shaving cream. Or as many cans as they have. Get the color-tinted ones. Go bonkers. Allow the 6-year-old to empty them all over your kitchen floor (assuming it's linoleum or tile). They can make sculptures, mountains, shaving cream drawings...whatever. then when you guys wipe it all away, you'll have a nice clean kitchen floor!