I will not allow W to take them and I will get the law involved if MIL tries to pick them up from school and take them.
As a matter of fact, I will be speaking to the school on Wednesday to modify our "parental release" to where it HAS to go through me on Monday's and Tuesday's. Then it will be the school's ass as well. I'm itching for a reason to bust those idiots.
The other angle I could play is the fact that MIL paid for the dance classes without consulting me. It's my night (legally.. got the court papers to prove it), so I can tell Grandma to pound sand.
Document, document, document.
This is something you need to nip in the bud. If they had treated you like a partner, I'm sure you could have worked something out. Your W & MIL want to do what they want whenever they want it. It sounds like part of the problem in your M was that you did not have firm boundaries--trying to be a nice guy. I suffer from that problem too. This kind of thing will happen over and over if you don't stop it now.
Your W's strategy seems to be to dismiss your plans as "unimportant." Try re-focusing the conversation on the inconsiderate act of scheduling something on your night w/o your OK. It doesn't matter what your plans are, the fact is that you have your own life, and they were rude and inconsiderate by intruding on it without talking to you ahead of time. It also sets a horrible example for your Ds of treating people inconsiderately.
Another 180--why does your W need to come to the dance lessons at all? They occur on your night...just as she scheduled them, so you can take them. It might be a place to meet some nice women. Having STBXW there will really cramp your style.
Now is the time to draw some lines. No matter what happens in the future, you will have to live with these people for the rest of your life. Establishing the rules of the relationship will make your life much easier. You don't have to be unpleasant or angry about it, just firm.