One of my professors once said he sometimes had students mount one
painting on another larger one to create a contrasting border which made
me think that you could paint needed size "mats" on old newspaper, trim
and mount the kids work on that. I'm lucky to have plenty of supplies at
my school, so I have my students work a lot on 12x18 paper that I've
trimmed to 11x17. When they finish they choose a 12x18 sheet of colored
construction paper, center their work on it and staple in the 4 corners.
Hope this helps you.
Linda in Michigan