Maple would be my choice of those mentioned.
How many youngsters will be participating? Roy and I and a few others prepped a bunch hickory backed hickory bows at my place one time for a big bunch of Boy Scouts. They weren't tri-lams, but they were quick to make, inexpensive, and easy to tiller. I also made a bunch of portable tillering trees on 2x4's, cradle on top, pulley on the bottom, for the kids to use, so they didn't all have to wait in line for a single tree. The portable trees can be mounted in a vice or ratchet strapped to a pole or tree, etc.
You can get clear hickory, usually at a local sawmill or hardwood supplier, of virtually any grain orientation as long as grain runout isn't severe, and back it ahead of time with a GOOD quartersawn 1/8" thick piece of hickory or maple, glued up straight or with a little reflex...add a handle block too, and they can take it from there. Is there a reason you'd prefer to do tri-lams instead? It just adds difficulty... perhaps unnecessarily for this apllication. Just curious.