I came across this idea a while back and I can not remember where I found it at.
Using a 1 gallon size ziploc bag, I filled it with a small amount of tempera paint . You can use one color or two. If you use two colors keep in mind that they will mix. I secured the opening with scotch tape to make sure that it did not leak.
You can give your children Q-tips to draw pictures or they can just use their fingers. If they use their fingers be sure to clip their nails beforehand.
This provided hours of fun and the paint won't dry up so you can use it over and over again. (Or at least until the bag starts to wear thin.)
Be sure to check out my Link Parties page to see all of the great crafts out this week.