How old are the girls? Have you ever seen the mother let the girls play outside alone while she watches from in the house? If the answer is no then I probably would say something to the mom. She also needs to know because of what the sitter might have been doing inside the house that she didn't want the girls to see. Maybe nothing, maybe even doing housework but maybe something else too. You might want to use that as your reason for bringing it up in case the parent had said it was ok for them to be outside as it is a way of telling her without bringing her judgement into play if she said it was ok. I am assuming the age of the girls makes it borderline for playing alone which is why you are concerned but all families have different ideas on what age is ok.