SOLVED. Mystery Book about Family moving into a new house that has a theatre in the basement and smugglers, at least 20 years old. [s]

[Nextra11]

Hi all, I need your help please. I've read this book when I was a kid (about 20 years ago) and I only remember parts of the plot. so it starts with a family moving in to a old house, in the basement there was a theatre with a secret passage that leads to the sea where smugglers smuggled something and the kids in the family tried to stop them. at one point I remember something about them grabbing one of the smaller kids and a shack with thornes, though that last part may be made up :)
If anyone has any idea, please!
Thanks.

[Kris]

Nextra11, can we eliminate Five Go to Smuggler's Top by Enid Blyton?

Google search of Goodreads:
site:goodreads.com "mystery" "middle grade" house theatre smugglers

[Robert]

Hi, I think that's The Young Detectives

[Nextra11]

Robert, OMG, I think you got it! Just ordered it and I'm 99.9% sure you're right!! Thanks a lot!!!