Yes, pretty much this.
If I was that affected by my pregancy that I couldn't go out, I wouldn't go to someone's house knowing they had planned an evening out, knowing I was going to reveal at the 11th hour I wouldn't be going...and also knowing that because I was announcing a pregnancy, they can't really say bugger all about it. It's very me me me.
No, @Sofasandslobbies you can't go out. It's just rude to leave guests at home. However, what she's done is very self indulgent and inconsiderate, but she's so self absorbed, she doesn't care about the effects on you. The princess is pregnant. Of course the world should stop. She could have made any number of faux excuse like PP said, when she could see everything being arranged and all the trouble you'd gone to on the group chat, "Money's a bit tight" or just simply "We'd prefer a night in with a takeaway, if that's ok with everyone else" etc and still done her big reveal without causing disruption. She didn't though, she watched you plan a whole evening out and arrange 4 children to have overnight childcare, which you haven't been able to do for a year, knowing it was all going to be cancelled, by her. And she knows if you don't act delighted at the fact you now have to cancel everything off the back of this, miss the one break you would have had in an entire year, or go round frantically trying to cancel the reservations you've made and the now non required childcare, she can do the whole "She didn't even care about my baby" which is not the case at all. It's the deliberate disruption. You'll have to get a takeaway now as well, as you hadn't banked on feeding everyone tonight.
Is there any way at all, that you can rearrange the childcare for another night. Have all the DC stay at home. If that means they wake her up at 5.30am, then that's on her. She's stopped you all going out as planned, with zero notice. So cancel the childcare you no longer need and keep it in the bank for this time next month. Apologise to inlaws etc for the last minute cancellation, but make it very clear, you've been put in this position by her lack of notice, and are equally annoyed by the cancellation, so if they would be able to offer another night, you would be incredibly grateful.
Don't invite her again. I'm so cross for you OP. We've only got 3DC, but I know how it is, trying to get grandparents to have the eldest, a qualified childminder or nursery nurse who does evening work to have the youngest (toddler twins, so you need someone confident to handle multiples) and paying for that, just to have a meal out. And making those two things align when everyone is free, and DH hasn't got to work late. It happens perhaps twice a year. We had tickets to a comedian, which were booked for over a year. That night, for reasons known only to herself DD refused to sleep, kept getting her brother up. Both went full feral. We literally couldn't leave. I know it wasn't their fault, but honestly I could have cried. Our one night out of the house, not being "mum and dad" in 8mths. Gone.
Cancel what you can, but please make sure you rearrange the childcare for next month instead, and have your evening out, for you x x