Oh my.......... I remember that age well.
I think I asked everyone that very same question as well! You know what response I got from a few people?
They told me that my children were just copying their learned behavior. When I heard that, I took a step back and tried to think before I spoke. Lo and behold, I was the culprit of that very same reaction!
I never realized how many times the word "no" came out of my mouth! No wonder why my children answered no when I would ask them to do something. I had to re-learn my responses to thier demands. Only then will theirs change.
Just as Bandit81101 mentioned, give them a choice, not a demand. Your children will then start to do that to you as well.
