Hi and welcome!
The first thing I would recommend is to take a giant mental step backward from how the kids' bm parents. I know that it's frustrating, but you really can't control how she parents during her time if the kids aren't in any immediate danger. It's part of the deal when people co-parent in separate houses. It would be nice if everyone was on the same page and kept things consistent, but the reality is that that rarely happens. I would focus on keeping things consistent and enforcing the things that are important to your dh and you at your house and find a good outlet for venting your frustrations about the poor diet choices at her house.
I'm assuming that there is a court order in place detailing the support payments. Personally, I'm a big believer in not paying anything outside of the order. The support numbers are based on income levels as well as what should be required to care for the children in question. It's not up to your df to send extra if she chooses to spend it poorly. I'm not sure what your CO says about medical expenses. Ours says that dh is responsible for 50% of uncovered, necessary and agreed upon expenses (like co-pays, for example). We don't pay anything without an original receipt on the advice of several attorneys.

If she doesn't provide one, we don't reimburse her for it. Granted, in our situation, she's been habitually dishonest and shady about the medical receipts from day one and has been deceitful in the interest of self-benefit in several other areas, so we don't trust her as far as we could throw her. If she does provide us with a receipt, we file it and include our 50% on her next support check.
If you don't have a CO in place and the support amounts are just random numbers they decided on, I would seek to have something put on paper. I don't know your financial situation and it may not end up being much different, but having something on paper offers every benefit in all aspects of coparenting life.
Does your CO include the right of first refusal, which means that both parents must ask the other parent first if they're going to need a babysitter for more than a certain number of ours. If it does, she may be asking your df first in the interest of following that guideline. If it bothers your df to have them when it isn't his time, he's welcome to say no.