Squirt blobs of paint into ziplock bags and seal the bags tightly!
Place a white sheet of paper under the bag and tape it securely to the table.
Ask your kinds what the two colors mix to become
Tip: try and remove all the air bubbles you can before sealing the bag!
idea from blog: http://hippiehousewife.blogspot.ca/2012/01/mess-free-finger-painting.html