I have smaller aquarium gravel right at the bottom of my tank, then a layer of bigger gravel on top of that at the land end, then a layer of even bigger river rocks on top of THAT so it still looks rocky, but the frogs can't really get at the small pieces. Do want more river rocks on top though, to doubly make sure none of the smaller pieces are visible/accessible.

EDIT: By the way, your FBT looks adorable