please say a bit more…
I’m a bit confused as to what you’re trying to accomplish by tumbling the glass. If you tumble the glass, then fuse, the effects of the tumbling would vanish.
If you give the kids a small square of glass to glue smaller pieces of compatible glass on to, you can use simple white glue like Elmer’s, then fuse. As long as the kids don’t overdo it on the glue it should burn off nicely during the fusing.
Good luck and have fun!