Near the end, when Tommy and Chuckie are going to leave Mirrorland, they run into the mirror and go through. When they land the mirror is flipped upside down behind them. When Didi finds them, the view chages back to them and the mirror is right side up.
They weren't in Mirror Land. They just thought they were because they had wild imaginations. And think about the other episode where Chuckie thinks he lost his eyes but he just lost his glasses. See what I mean? There are so many of those kinds of episodes that if I listed them all, this page would have 5 megabytes of text!
I think you might be missing the point of this post. Obviously it was only their imagination; it's impossible to watch this episode without coming to that conclusion. It's just saying that the mirror's wrong side was showing.