A.G.
How old are your children? Will they be in school during the day? That would impact how much to offer her. Happy birthday!
I was thinking $300 also, $100 per child. You aren't actually paying her since she is a good friend and you are leaving food and money for incidentals, but you don't want to take advantage of her either. I think $300 is a nice gesture. If she has to take off work, it may need to be more.