Here's what we ended up with at the end of the project. :-)
I started off by putting two splats of paint on the paper and put the balls in the box. I picked up the box and shook it around to show Kellan what he could do. He shook the box a little and then stuck his hand in the paint. He didn't like having it on his hands very much. So I wiped them off and gave him a brush and he was much happier. This sort of surprised me. I really thought he would get into finger painting.