What you needAcrylic Paints
Large Line Art Drawings (clip art)
Print, draw or photocopy some clip art or drawings. Tape the picture to the back of the glass window and have the participant color in the picture from the other side.
This will give them a guideline to follow and will teach them coordination.
Any mistakes can simply be wiped up. Once dry a scrubbing cloth will remove the paint from the glass.
Alternatively you can divide the window into sections and have each participant paint their own block.