I've been meaning to do this one for a long time. It's an idea I got ages ago.
We used picture frames with glass, Plaid's lead liner from their Gallery Glass line (it's for simulating lead outlines, and it isn't lead), Biocolor paints, and drawings on paper that the children made.
First they drew their pictures.
When they were done, I took the glass out of the frames, and carefully traced their lines with the liquid leading.
When it was mostly firm, I had the children paint their drawings, showing them how the paint would 'pool' inside the borders of the leading.
When dry, they were inspected to see if any spots needed more layers of paint,
and when they were satisfied, we put them in frames, and then in the window!