My suggestion would be to stick with two-reed calls of thinner material such as "proph" .0025 and .003. and stretch the material about 1/4" (sorry,...don't know what the would be with a tension meter). A little back-stretch is okay, but not necessary. Stick with either a youth frame or a small adult frame. A good "beginner" cut is a center V-cut,...and that cut can be modified to a combo, batwing, or ghost easily so that the user can have options to find out what works best for them.
If you want a specific construction for a call: (youth frame for little guys and small adult for older kiddos) both reeds proph/.0025 with approx. 1/16" stagger (top reed longer), approx. 1/4" stretch, slight back-stretch if desired, center V-cut to start (depth of first cuts approx. 1/8",...deepen if needed)
