Why do you think you would lose your superhost status. You aren't cancelling they are.
It is ridiculous for them to book to stay in a listing which has pets when there son is allergic.
Message them back and say you are sorry they have decided to cancel as their son has a pet allergy. And that it is shame they booked a place that has pets knowing their son's allergy.
Confirm that you were there when your cleaner cleaned the property and know it was left spotless. And that you even offered for your cleaner to return, which they refused.
In your situation I would advise the guest any cancellation will be in line with the policy. However as a gesture of good will on your part, if you receive another booking for the same period, you are happy to consider a partial refund.
Make sure you call Airbnb and log this issue with them so they are aware of the situation. Stress how clear you make it that your listing has pets and that you were present when the cleaner, cleaned the property to her usual great standards.