They are certainly taking advantage of you. On the day the school aged child is in your care for a full day you need to charge the same fee as the sibling.

I too had a family take advantage of me in the same way. When I first started my daycare I started with 1 boy. I charged $25 a day. When I started taking other children I increased my fee to $27 a day. The family ended up suffering a terrible tragedy. At that time I agreed to take the older boy before and after school. Feeling bad for them I only charged $2 a day (for 2 hours) for that child. I ended up increasing that rate to $3 eight months later and eventually to $5 a day ten months after that. I also had increased my full day fee to $28 during that time. I gave the family a break and for the most part took the oldest son during days off school and during the summer. The one day after 3 years of care they totally out of the blue gave their notice and in 2 weeks were gone. No gift, no card, no thanks. I was hurt. We ended up running into them a month later at the campground and all that they did was say Hi and keep walking. I was crushed.

Sad to say but we have to keep our heart out of it when it comes to the parents. Don't do them any favours because they obviously don't do you any.