I hate questions about my childhood. Maybe hate is too strong a word. I dislike them for sure. Why? I just don't remember as much as I probably should!
So, a secret hiding place as a child...not that I can remember. I read a lot. So maybe, if I had to give an answer, my hiding place was in the pages of a book I was reading. It would allow me to "hide" where I was at and go to a different world in a different time (I read a lot of the Hardy Boys growing up).
For a more specific answer I will need to defer to my parents.
What about you? Can you remember a secret hiding place as a child?