This is the main idea because in the beginning of the book
when the queen came out, Mimi almost trusted her into going into her
world. When she refused, the queen took her dog and went back to
her world. If Mimi took her hand she would be stuck in the other
world and be trapped forever. Even though the queen promised great
things Mimi still didn't do it. Also, in the other world there was a
suspicious man who said he would help but Leo didn't trust him and
they escaped him. This is why you shouldn't trust anyone.
Another example of this is when the guy they met told them that he
knew where they were from and told them that he would help them get
her dog back from the queen. But as they began to trust them he
kicked them into the forest. Now right where I am, the guy they
trusted is hunting them down to turn them into the police. This is
why I think that the main idea is that you shouldn't trust anyone.
In conclusion, I think that Key to Rondo tries to show you that
you shouldn't trust anyone. There are three examples of this in the
first half of the book. One was with the queen, the second was with
the suspicious man and the last one is the one who knew they needed
to get back the dog. They were different people but they had one
thing in common: they all betrayed Leo and Mimi. This is why I
think that the main idea is not to trust anyone.