Aww that is very nice of her! It's entirely up to you to decide if you want to accept it. If you do, I would give her a receipt for it.

I personally don't ask money for taking them on field trips. I pay for lunches out, movies, playtime fees etc out of pocket. I feel that I charge enough to cover these costs. I have had a few parents offer but I always decline. If I did it daily or even weekly I may reconsider. Right now, I only go on a field trip that costs money maybe once every 2 months I miss my smaller group and summer!!!