Legal Question: trampoline in backyard of rental property

[Catherine Brennan]

Good Afternoon - seeking advice about the following:

I drove-by my rental property and saw there's now a trampoline in the backyard.  I read through our lease again and tenants are required to get permission/authorization to install items like this which tenants didn't; and there is a sentence in our lease that we are not liable for any issues with these additions.  

Anything else I should do to protect ourselves?  They've been good tenants so far.

Thank you, in advance, for your help.

Catherine