OP, you seem to be stuck between a rock and a hard place, but step back and make changes.
The dynamics here are flawed. Your sil, taking money for doing you a simple favour, is very wrong, but very telling: family members shouldn't do this.
Your DD, as so many young people, is struggling financially. You're right, she still wants the champagne lifestyle on a lemonade budget. This is, I think, chewing herself up inside. She resents your comfortable lifestyle, and is used to you bailing her out. Her only weapon against you, because she can't match you financially, is to lash out, as she did, and withdraw your generous childcare.
She's doing this to 'punish' you for not giving her the money she feels she was promised. Deep down she knows that the childcare you provide will cost her a fortune elsewhere, so she's squared this with herself by telling herself that because you naturally love dgc, you see it as a pleasure, not a chore.
She has now backed herself in a corner; she knows she's reliant on your free childcare, but her pride will make her defensive.
I would do nothing. I would leave the ball in her court. At some point she will have to apologise and either ask you to continue childcare, or she will make her own arrangements, driven by her pride. If she takes the latter option, it will cripple them financially, so don't be surprised if she asks you to contribute financially. If so, refuse. Then have an open, calm discussion about finances, what you are prepared to do, if anything, what you expect in return, which is respect as a minimum.
This is an opportunity to, once wounds are licked, clear the air and start afresh. You need to fully understand how stressful being constantly short of money is for them, and forget about how you struggled. She needs to grow up, live within her means, let go of resentment, and be truly grateful for what she has had from you.
Tell her calmly that you are hurt and listen calmly to her side. Then put it aside, set up new boundaries, and move on. It will work, or it won't. But if it doesn't, at least you can look back and say you handled it fairly.
Good luck.