Coffee filters make a great canvas for creating color washes that you can incorporate into collage and mixed media designs, and they are easy for kids to work with too. When my kids and I did these we just used markers and a spray bottle filled with water. We used the markers to color on the coffee filters (no specific pattern necessary and scribbles are allowed), and when we were done, we held the coffee filter on the outer edge and sprayed the water on it.
It does get a little messy as the colors start to run when you are spraying so it's a good idea to work over a sink.
I've experimented a lot with coffee filter designs even for grown up projects. It's always a lot of fun.