Claim your expenses on your damage deposit, but note the short time frame you have for making a claim.
Contact your guests through the airbnb message board and confront them ?
From the photos, your coach seems to have loose covers - I suppose they would wash or dry clean ?
Claim it on your own insurance ?
Kids are messy in general, so washable slip covers are very handy to have. If you are doing back-to-back reservations and accept families with kids, having spare slip covers could be a practical measure in order to be prepared for future incidents of the same nature.