Help with money regarding child care

Hello I need advice,

My ex and I broke up last may, we have a 3 year old and a 1 year old. When we were together my father would watch the kids but in the last couple of month my father has been in not good of health but does me a favor to watch the boys when it’s my day. We asked my ex to find child care for the boys on the days that he has them. I give my father $80 for the 3 days that he watches them he does not give me any money for that. Well he complained to me bc he he had to pay $40 for child care on one of the days that he has the kids and is demanding me to give him money for it. I told him that I couldn’t and that he could. We pretty much have the kids 50/50 and take care of what they need when they are with us. Do you think I should help him or is it okay for him to pay the money since it’s his day of the week?