You begin by cutting shapes out of sponges. You can do it with scissors but it’s hard for small hands to do so your grandkids will need help. I got a package of 8 sponges at the Dollar Store
Cover the work space with newspaper. Squirt acrylic paint onto a paper plate and let the kids press the sponge shape in the paint and stamp on their paper. I used pieces of brown paper sacks because it’s thicker and the paint won’t go through to the other side.
You can also make an accordion shape book or use your imagination to create something special.