No Pets in Previous Lease

[Geri May]

My husband and I purchased a duplex, took on the two tenants that were already on a contract through the previous management company. Flew to Texas to take care of some business and begin a flip on a single-family home we just purchased and I reached out to the tenants so that we could stop by and introduce ourselves, heard dogs barking in each unit so after returning home and looking over the contract I found they should not have pets.  I am not wanting to take away someone's pet or charge them an arm and a leg, but one tenant has responded to my email and said she must have "overlooked that in the contract". She has a child and I don't want to take his pet away, but according to the contract there must be a written agreement between the landlord and tenant and we do not have this. I am reaching out for some guidance on this. We have other properties and do not allow pets.  Anyone have a pet policy I can use and do you charge for breaking the lease, it wasn't under me so my heart says no but to just collect a pet deposit and/or monthly fee. What do you all do? I love having experts weigh in and provide guidance. I don't want to be a hard ballplayer, not my style, but don't want to just let it go either. 

I appreciate your input!