I am a K-8 Computer Teacher and I love using Monster Mash with my students. Currently I'm using it with grades 2-8 along with three.js to let them make their own 3D game characters and with K-1 to let them turn their drawings 3D!
I tend to have the kids make a transparent PNG and then import it as both the template image and the mask, however, finding the import menu and doing the import process twice was challenging for the little kids. I edited the Javascript and HTML a bit to make a version with a single import button not hidden by a menu: https://monster-mash-optimized.vercel.app/
I'd love integrate MM with the three.js editor and Kleki to make a website that allows kids to quickly make their own games using hand drawn assets. I'll keep you all updated here!