@Alison219 It sounds like either the guest sent you a cancellation request that you accepted, or the guest called the Airbnb customer services and they wrongly did a host-intiated cancellation for them.
If this was an instant booking, you can cancel penalty-free if your guest discloses that they are going to break your rules - in your case you have selected that your property isn't suitable for anyone under the age of 12. In future, I would suggest you just use that option rather than involving Airbnb so mistakes don't happen. In this case, I would contact Airbnb service as soon as possible and ask for the penalties to be removed, as you didn't do anything wrong here.
I might add a note to your other things to note and rules section stating why your place isn't suitable for infants, and that you only host a maximum of 2 people, and that includes guests of all ages.