I'm working Wednesday morning and am kind of happy to be unavailable for her, not the kids. Is this wrong?
If you're asking if it's wrong to feel happy that you can give her an excuse for being unavailable......no, it's not wrong to feel that way. If you are asking if it's wrong to be unavailable Wed morning.....no, it's not. But even if you did not have a legit excuse, you should not feel bad about telling her you're unavailable. She may try to impose a sense of "responsibility", but as you said, she will have to discover the bad side to these decisions.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!