@Sherri17 sorry to hear that 😞
Asking for extra cleaning fee is basicly the same procedure as a damage claim. You should take pictures after their stay before cleaning, first contact your guest to pay for extra cleaning, if they don't want to pay then after 72 hours you invlve Airbnb to charge them from their security deposit. If you didn't take pictures and guests don't want to pay then your chances are very slim I am affraid.
Maybe it would be the best if you contact your guest in a very polite and gentle way and hope they will have mercy and pay for cleaning. English is not my native language and I am sure you could write it better, but maybe something like this: "Dear XX, I hope you felt comfortable at our place and had a good time with your friends. Unfortunately, after your stay , our cleaning lady needed more than average time to clean everything and prepare the apartment for our next guest. Would you be willing to help us pay her 50$ for the additinal cleaninig time needed ? We would really appreciate it very much. Thank you in advance 🙂 "