They'll be able to figure out something is up. Be honest.
I really don't think they would appreciate you going into debt to accompany and entertain them for their trip. They are probably more concerned with how their new grandchild is doing and his/her longterm care.
Maybe they will help you out a bit, stock your fridge and maybe you can learn new recipes from your MIL.
Think about how you would feel if the situation were such that you were visiting your new grandchild. I would certainly be more concerned for the welfare of the child vs how my daughter was entertaining me when she can't afford it.
Stay out of debt.