We're on vacation and it's raining :-( My kids want to paint some pictures, but unfortunately, we didn't bring pencils with us. My laptop has multitouch and we started using Paint.NET, which works quite well. It's fascinating for them that you can undo mispainted strokes and that you can paint over already painted areas.
But Paint.NET has some disadvantages for children:
The color picker makes it hard to pick dark colors. My son would prefer e.g. two color wheels, one with white in the middle and one with black in the middle. Of course, it is possible to pick dark colors in the extended view, but it's too complex then.
The eraser erases to transparency. Transparency seems to be hard to understand. Transparency does not need to be supported.
Paint.NET's tool windows often overlap the picture, so you need to move them away. I guess it would be simpler to have the tool windows docked (which is IMHO not possible in Paint.NET)
The object modifier points are too small and hard to touch in Paint.NET. This should be more touch-friendly.
Ideally the UI would be configurable so that I can remove everything that I have not explained yet.
Such a Paint.NET alternative would need to run on Windows 10 and not be a Windows Store app (I don't have and I don't want an account)
File formats are not important. I guess it will support at least one common format.
Undo/Redo in virtually unlimited number is a must-have.