One of my all time favorite techniques to teach is tissue paper bleeds! There is a lot a kid can do with a page full of color! And using tissue paper and water can give you some really breathtaking results!
Every time I have ever taught a student how to create a wash of color on their paper with tissue bleeding, they've taken their new colorful paper in completely different directions. Some turn it into skies, some turn their rainbow paper into the sea, some use it for a background for a self portrait, or typography quote. Endless possibilities!
Here is a quick demo video if you want to do this activity with your kids or students!
Watercolor paper or mixed media paper
Tissue that bleeds (Probably not gift wrap tissue)
Cup of water
I can't wait to see what your young one's come up with!