Live action magical/adventure movie

Hey i’m looking for a wierd movie, it must be around 20 years old or more. I remember being a group o kids (live action) living in an old mansion near the see (the sea was kind of the backyard of the mansion). In the back yard in that beach was a magic creature done with a puppet that live inside an oyster or maybe a turtle shell (the puppet looked like a little old man aswell lol). That creature was magical and could give wishes to the kids in the mansion.
In the mansion was also living a bad and really intelligent kid (the bad guy), he wanted to kill the magic creature and do experiments on him to know why was he magical.

five children and it