my thoughts.... get a custody schedule in place, you get them half the time, she gets them half the time, the other half of the time, she has to put on her bgp's (big girl panties) and deal with being a single parent just like you will have to, those are my thoughts on this.
You are right, she didn't ask or consult you in her decision to break up the family and you won't be her "babysitter", you are now a single parent just like her and you will have to learn how to deal with that situation, just like her.
Yes you are saying NO to her and not the children because when you have the children based on the custody schedule, you don't plan on contacting her to watch the kids when you need help, you will find your own way to deal with this.
If she really wants freedom from you and the relationship/marriage, well then give it to her and everything that involves.
You can't be her safety net and rescue her from her adult responsibilities, if she wants to live on her own, she has to deal with the full reality of that situation.
You're not being mean or punitive, you are being REAL with her just like she's being REAL with you. You've had to deal with REALITY, it's ok for you to let her deal with it also.
