I have a similar relationship with my mother-in-law and frankly it's because I am DONE. I made an effort early on, but there's a point where you come to the conclusion that it's not worth it anymore. I don't know your circumstances, but I can speak a similar point of view as your DIL.
How to handle the behavior? Stop trying to make it something it isn't. Your DIL is inviting you to these things because she realizes that you love your grandchildren and your son. She is asking you for their sake, not hers. Trust me, she could be crueler and just lock you out from all family things altogether.
If you haven't apologized for the texts, you should. But that isn't going to change the fact that she will always know in the back of her hear what you really think of her. Focus on your relationship with your son and grandkids. Be polite, but don't force yourself into her life.
If you feel it's a burden to care for your grandchildren, don't do it. Only charging them $30 a week is generous-if you're a stranger. But you're their grandmother. There may be a point of contention there. You resent that they don't appreciate you more, but in their mind they are paying you for a service and shouldn't have to show more appreciation.
Again, I am speaking from the perspective of a DIL who has all but ended my relationship with my MIL. Sometimes personalities just don't mesh. Be polite, model a civil relationship for the children and move on. Maybe there's a chance for reconciliation at some point, but it's not happening today.