Tenant got pet after move-in, not permitted

[Nathan Mount]

How would you handle this situation?   My tenant has been in the unit for over a year.  No pets permitted, per the lease, and we just don't allow them in any of our rentals.

It's come to our attention, that they have a dog, and to make matters crazier, we received a hand written letter from the tenants 10yr old daughter, demanding we allow pets.   They don't know that we are aware of this pet yet.

They're on a MTM lease, and I can probably increase rent by $100/mo if they move, but I don't think they're wanting to move soon.

How can I address this with them tactfully?