1. You can see through the peephole to make sure rules are respected. If they aren't, you can inform adults, like teachers or parents, and they will︀ punish the kid who has your key for breaking the rules. The adults may also︁ recover your candy (or part of it) and give it back to you.
2. Same︂ as above, there are documents with rules that you and the kid with access to︃ the chest have to sign and follow.
3. It depends on the documents and who︄ the trustee is. Sometimes the kid with the key works for a group of bigger︅ kids, and they will just give the key to another one. In other cases, the︆ trust documents may mention who the successor trustees are. In the worst case, legal proceedings︇ would take place accordingly.