With my son I made a wooden shield, glued canvas over it and then he painted the canvas with his sigil. We used poster paint (children's paint) because I remembered I could not completely wash it out of clothing.
However there seems to be wiggle room between 'not coming out in the laundry' and 'being fixated to a DIY project'. The paint does come off when wet, not completely, but enough to stain seats and clothes.
Could I use regular wood glue to fixate the paint? Just applying a thin layer? My other option would be varnish.
The main point is that the shield will be in use, so whatever I apply cannot be brittle. It should stay in place when the shield is hit by a wooden sword (obviously).