Remove any, add your own, and set how often each should be reachable. Choose prizes you can genuinely hand over — the promise only works if it is kept.
During setup you can pick 5 everyday prizes plus the one big thing they keep asking for. If you try to add more the app tells you right away, and nothing you picked is quietly dropped.
There is also a separate list of prize ideas your child can choose between. You fill it; they pick. That way they never choose something you would then have to refuse.