Find some sort of clip to attach to you child’s key ring. I used one from an old back pack. You can buy clip sets like these at most fabric stores. You could also use a carabiner or something similar.
Open up the front pocket of the backpack and attach the other end of your buckle or some sort of loop to hook the key ring to. You might need some webbing or twill tape or ribbon or something.
Secure and concealed (except for the fact that the entire internet now knows where to find Macy’s key) and almost lose-proof!