Helping and enabling is a fine line and one that is easily crossed.
I think the others have offered some great insight. If it were me, I would have a conversation with her. I would ask her what other options she has for grocery shopping, Dr's appointment's, etc....... This will allow her to take some ownership in her life. I would let her know that I can help once in a while but cannot always be available and it's important for her to have other options in play. Tell her you worry about her and the kids and want to make sure she has other options. I always use the "you never know, I could get hit by a bus" line. Try and find out what's available in your area so you can offer some resources.
In order to keep things good with your grands, I would not completely cut her off of help but I would start to say "I can't help you this time, maybe next time". This way you are leaving the option for future help open but also holding a boundary.
