UPDATE:

So I talked to her mother and let her know what I found. She was great about it, she was disappointed and used the word "sad" to describe how she felt. She said that she would ask her her side of what happened and let her try and explain what happened. We both agreed that it was likely a situation where she had a couple friends over and it got out of hand.

I just received a text message from the girl and she was EXTREMELY apologetic. It seems very sincere and it's pretty lengthy. Some quotes:

"I don't know what came over me, I am ashamed".

She says that she only had 5 friends over to celebrate one of their birthdays and "apparently I don't know my friends that well". She doesn't want us to pay her and wants to pay for any "damages". There aren't any damages, but now I'm wondering if she knows something I don't.

She's going to come by with the key and I'll get a chance to ask her about what happened. She said she realizes she abused our trust and it makes her upset because she really likes our dogs and our daughter.

Thanks for the help/advice everyone. Especially you insom, I essentially used your approach and I think it proved to be the right one.