It will make you feel better. (Please note sarcasm) Didn't you know that the prevailing thought is that their child is the center of the universe!!!!
Seriously when will people realize 99.9% of teachers go into the profession because they have a desire to help children and feel that they can make a difference in their lives.
Tell the parent you do and ask her to look through your eyes. Does she ever have 25 kids to take care of at once, and to teach. Invite her in to teach the class a lesson on something she would like to share. Then don't support her. I think too often parents who haven't been in a school for awile don't think what its like at all. I have had volunteers come in for just an hour and tell me they didn't realise what teachers go through.
You look through a child's eyes whenever you plan lessons and try to figure out what went right and what you could do better. The real world doesn't look at just a child's point of view. I'd like to go to some of these parents' jobs and give them some "great" advice. I wonder if they would appreciate it.
It's never a good idea to answer a communication that upsets you right away. Wait 24 - 48 hours before you even think about doing something. That will give the parent time to think about what THEY said in the heat of the moment, and will prevent you from putting something in print that you'll regret later.
I agree with the poster who said it would be wise to wait before responding. I also find it helpful to invite the parent in to discuss the concern. Many times, things seem to come across in an e-mail in a way that is more nasty than originally intended. What specifically was she complaining about? You can remind her that you are an adult and sometimes it isn't safe or practical to look through a child's eyes. You have to use your adult judgement. If I ran my classroom on the whims and desires of the kids, no one would be learning very much. This doesn't mean that you aren't child-centered. You're right--we often must make decisions for the good of the classroom community.