ABANDONED. Possible YA written prior to 2000 about brother and sister who find door in cave that leads to dragon world?

[Katie]

My husband is trying to remember a book he read back in the late 90's. He is not sure when it was written, but he feels it was more young adult versus a little children's book. The plot is about a brother and sister who are on summer vacation (at a beach in England?). They explore a cave and find a hidden door that takes them to a dragon world, where there are green and black dragons at war with each other. There is an old black dragon that lives with the green dragon clan though. Though the kids think they have been in this world for months, when they go back through the cave door to their own world they have only been gone for minutes. The next summer when they go back to the beach for vacation they go to the cave and find that around 200 years has passed in the dragon world. Please help!

[Kate]

Katie ~~
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Good luck

[Pixiegirl105]

The Forbidden Door
Sounds like this one I read when I was a kid.

[Kris]

No response, moving to Abandoned folder.

Katie (OP) was last active on the site in March 2019.