S.M.
Don't let it go. You are charging almost NOTHING to care for these kids, and you are entitled to have a life, too. You should make clear to the parents that they must pick their kids up on time. (Dropping them off on time is probably less of an issue.) My late mother-in-law had a childcare, and after many years of bad experiences with parents who were always late, her rule was that for every five minutes a parent was late, it was an additional $5.00 charge. Parents were required to pay the childcare bill every week, and she would not allow them to bring the children into her home unless they had paid the previous week's bill and any late charges.