I found that Doodle Buddy for iPad – Paint, Draw, Scribble, Sketch – It’s Addictive!
By Dinopilot would be appropriate for students in a PK-Kindergarten classroom. The use of fingers gives them some of the feel of finger painting, without the mess. Unfortunately it has a feature for erasing when shaking it, that at this young age especially, could happen easily. So I think I'd need to bolt the ipad down somehow, lol.
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