I am looking for a book I read about 10 to 15 years ago.
The protagonist (a girl?) and their family end up in an abandoned airport or military airbase, I think some ghost planes appear and then they were in a fantasy world where they could be whatever they wanted to. If I recall correctly the mother became a mermaid and the father a thought or water sculptor.
For some reason the protagonist buys none of this and goes to a castle but the closer you get to the castle the smaller you become, so if you try to approach the gate at some point you would disappear. They end up getting inside and into a pool, but it was not a pool but a pot meant to boil kids for the thing that was "dreaming" the whole thing.
I think the book belonged to a series of YA books but I am not sure.