@Bonnie23One a month, I will need to have the mattress pad washed anyway. I would not charge for washing mattress pad and bedspread.
A cleaning of mattress needs to be extra since if it is not clean, urine odor will be on the mattress for some time. And it is not good for next guest to stay on a mattress with pee. You may get a quote from cleaning service. And remember to keep photos and send the quote or invoice in the Resolution Center. You may or may not get the money depending on Airbnb CS's discretion.
If you did not warn the guests when they check in about peeing on the mattress because they have children, you may give them excuse. Otherwise, you may want to seek the compensation.