I had my mom and her boyfriend move in early this year, for a few months!

They're still here. The only thing I can suggest is just come out and tell your daughter she must pay you a certain amount each week for room and board. Then, if you can, you can set some aside to help her when she moves out. Also, give her a deadline as to when she needs to be on her feet. Seriously, if she is only working 6 hours a day, she can either help with the kids more or get a second job.
Why don't you talk to the grand kids other grandparents and work out a co-child care situation. You seem comfortable when they have the kids. This may help you take your trip to NYC. I don't think you need to cancel your trip. That would not be fair to you.
Trust me, start setting some limits and lay out a flat rent. That way, there will be no hard feelings and everyone is on the same page. It's great advice from my most respected friend. It worked for me!

No matter what, good luck and enjoy the time you get to spend with the grandchildren.
We are here for you if you need to vent some more. Some of us really do understand with some first hand experience!