Some crackers have plastic keys in. Unfortunately they replicate an actual key blank that will fit into (but not unlock) many door locks. Here’s one we retrieved earlier.
Your darling child or (evil little horror) is just itching to try it in your door lock and snap it off inside. That’s probably better than them swallowing it though.
Leave some mince pies and a glass of milk if you have to call an emergency locksmith. Ho, ho ho!