Do I Have to Pay?

So over a year ago I left my ex and moved out with our kids. Recently we mediated through the courts and determined 50/50 custody and splitting reasonable joint expenses. My ex has to this day never paid support, that is supposed to start this month. However, he is trying to get me to pay his daycare expenses.

This wouldn't be a big deal, except the "daycare" provider is his sister. He provided receipts from his mom, and they were $200/300 more than even licensed daycare in the area. When we were together, his family never, ever charged him daycare.

Also, I'm home 24/7 on maternity leave, and have offered several times to watch the children. He's just being stubborn, and refusing to allow it. Even though where he takes them now is farther away than my house. I have also expressed major concern with his family watching them, as they believe in TV 24/7 and unlimited junk food. My oldest since we separated has had FIVE, yes FIVE cavities and my youngest has had TWO because they give them chocolate, pop, candy etc etc all day long. My youngest also said that one of his family members pulled her by the arm and hurt her for having an accident. They come home in flea bites because they don't flea treat their animals, too.

Do I really have to pay for this? I'm talking to a lawyer next week, but I'm freaking out right now. I don't have the money to pay this, it's more than his child support payments by $300.