This is another oldie but goodie. We have done this before as a science experiment alone.
Pour about a cup of milk in a bowl.
Drop a few drops of food coloring along two sides of the bowl. Add a few drops of dish soap in the middle of the bowl. Or, instead you can dip a toothpick in a little dish soap and have them to put it in the food coloring in the milk.
Either way, the reaction of the soap disrupting the surface tension of the milk causes the colors to radiate away from the soap.
We then decided to make some colorful swirling colors paper (similar to the shaving cream marbled paper activity) by dipping heavy paper (we used watercolor paper) in the colorful milk swirls and then pulling them out, and then just letting them dry.
They turned out quite well and will make nice note cards or backgrounds for their school work journals.