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The teenage girl who is there becomes more and more withdrawn before she disappears (turns out she changes into a fish)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 I hope somebody else can help. Bumping for you.

Thanks but the detectives in the section found it on the 4th post.

Thief of Always? I didn't read it but searched some keywords on Google.