We rent our house so can't do much outside, but currently we have a stabbed area and a sandy/gravel area. There is a very thin layer of gravel so we were thinking of raking it up and putting bark chipping down so our toddler can play out more safely. The issue is cats using it as a giant litter tray, we have two and seem to attract all the neighborhood cats, at the moment they do use the gravel. Can anyone suggest either an alternative surface or a cat detergent that doesn't involve spraying chemicals everywhere?