D.R.
It is so nice when they really want to be there for you. Enjoy it while it lasts, cause in just a year or so away it may all change quickly. If you really want to show her how much you appreciate her help and show her it is ok to go and have fun to here is an idea that may benifit the both of you. See if you can get a friend or relative to watch the younger children. Then start with a mom and daughter date early in the evening kinda like around 4 or 5 get the best dinner deals. Go somewhere that you both like or she does kinda a grown up place not like McDonalds, or even try a new resterant. Discuss how important she is to you and how she plays an important role in your family plus explain that you love all the help she gives you but everyone needs some down time/ me time and it is ok. Remember you have to show her that as well. Then pre arrange for some of her closes friends to meet somewhere like the mall or to go miniture golfing something they all like to do. Have it a surprise for her and just let them have fun and just step back and watch or heck even join in if she isn't too "embaraced". lol. Good luck and enjoy yourselves.
Actually I am going to have to do that with my 10yr old son.