This is a book the children enjoyed. What's the subordinate clause and it's introduce by what?

A subordinate clause is also known as a dependent clause. It contains both the verb and the subject and commences with a relative pronoun or a subordinate conjunction. The dependent clause doesn't convey the complete meaning of a sentence.

Hence, in the sentence "This is a book the children enjoyed."

In it the subordinate clause is "the children enjoyed" and is introduces by "this is a book." Which acts as independent clause conveying complete meaning of the sentence.