What's the name of this children's book from 80's or 90's

[frenchnoodle]

Mysterious demon type thing takes a boy to a magical mystery house through a portal in an alley way wall.
There is a nice old lady there, plus 2 or other kids.
The teenage girl who is there becomes more and more withdrawn before she disappears (turns out she changes into a fish)
Over the course of a week all 4 season happen.
There is a Halloween tree with masks on, each mask turns you into that monster, so the vampire mask makes you fly and drink blood...
The main character realises it's actually more like a prison and when he escapes 10 years have passed, another boy escapes with him goes home and comments about how old and fat his parents are.
Turns out the house is "stealing" the children's life.

Anyone know what this is?

I don't know but it sounds intriguing! Did you get the feeling it was English/American/another country?

Could it be this?
en.wikipedia.org/wiki/The_Thief_of_Always

Clive Barker - the Thief of Always

Yes that's it!!

Thank you, I wanted to get it for my 7 year old.

How did you find it, I've been using all kinds of search terms for about a week now?

It sounds terrifying 😱

My all time favourite book, the Thief of Always! :)

Clive Barker the horror writer? For a 7 year old?!

Honestly it's a kids book.

It's no more scary than Harry Potter, I thought it would be a good follow on to read with him.

something like 'children's book halloween tree girl turns into fish'!

It was the only children's book he ever wrote. I think I was around 8 or 9 when I read it.

It's a shame as from what I remember it's a good book.