I'd consider raising crickets or fruit flies. (I breed crickets because mine can eat full grown ones, but if I'd kept a Spring Peeper I'd probably still use crickets just for simplicity)

They breed like crazy but it takes a while. The main problem is that you'll have to have them of various ages so they be hatching frequently. The other problem is it's a bit hard to clean the cages with a bunch of little ones running around. However they can't climb up the walls minimizing the chances of escape and maximizing the chance of recapture should one manage to get out.