Ok, I feel I need to give a bit more info in my defence here!
We had the original conversation on Sunday, during which my knee jerk reaction was "oh no a weekday, we've just had snotty letters about attendance being cracked down on, I'm not sure if the school will authorise this". I genuinely had no idea if they would or not, but the letter suggested they wouldn't. MIL wasn't particularly worried, insisting they'd see the inportance of a wedding, and wasn't really listening to our explanations of the new rules.
That evening DH and I had a chat about possibly it being for the best if they couldn't come due to school not authorising it as we wouldn't have to worry about upsetting FIL to be, who is ok but 30 years older than MIL and a bit of a victor meldrew character, who has fairly regular strops if things aren't done to his liking. It seemed to us that he had engineered the day so as to exclude as many of MIL family as possible by having it on a weekday, meaning it would be difficult for nieces, nephews and their children to attend as they live quite far away. We hasn't made up our minds whether or not to bring the kids for this reason, or whether to just take them to the dinner afterwards.
If we did decide to take them out of school, we had thought it may be easier just to go with "dentist appointment" rather than apply for permission and be refused, but since wedding is not til October we thought we'd see how others got on in the meanwhile with applying for absences and see closer to the time.
Next day, MIL phones me to say it's ok I've spoken to the school and they've authorised it. Giving the impression she had actually mentioned names, dates, and been given official permission. Until I came on here I didn't realise that wasn't possible so didn't call her up on it. After reading a few replies here saying she wouldn't have been able to do that I asked her again and she admitted she hadn't got official permission but the lady had said it should be ok.
This was the very next day so I'd had all of 2 hours in which I could have asked myself. I felt she'd jumped the gun and removed my chance to "lie" to the school about the dentist, if that's what we chose to do. She does feel as if she's someone with the gift of the fab, for whom people will do anything if she asks them, and so now she seemed a little smug in that of course the school had given her permission when they would have been very unlikely to do so for anyone else.
Our children will be 6 and 4 at time of wedding. Not likely to make much noise, but not likely to need to be much noise to make FIL cross.
DH had felt this was a good plan, was happy not to do anything yet, and didn't see the issue with them missing ceremony as long as they could go to the dinner. He was equally cross she had done this, as it just smacks of mummy stepping in and sorting out his problems, which he hates.
We do like FIL for the most part, but suspect MIL would be happier with someone less grumpy as she is quite young and out going at heart and he seems to age her. However would never say this as it's not really out place. He's certainly not abusive, just a bit of a misery guts.
MIL originally wanted a nice big family wedding on a weekend with all extended family, but it's gradually been eroded into 20 people at the registry office on a Tuesday. I feel a bit sorry for her, I understand why she wants her GC there but I probably am a bit cross that, once again, she's let everything be downgraded to something she doesn't want in order to please FIL and that if she was willing to give up all her wants, it kind if makes us think maybe we should go along with it too and not bring DC? It's like she wants us to be the ones to make a stand and bring DC when she has given up all the other things she wanted to have. This way it's us getting blamed and not her, IYSWIM? The stubborn side of me wants to bring them and give them a bag of party poppers, but I don't know if I can face the glares and snide comments forever after.
Sorry that's so long. Does that answer everything?
Genuinely thank you for the replies. You've made me think about my motivations in this. I think I'll explain all the above to her (minus not thinking they should even be getting married!), and see what she says