How do i turn this fragment into a complete sentence. the children in school?

Alright, let's analyze this.

"The children in school."

We have an article ("the"), a noun ("children"), and a preposition and prepositional object ("in" and "school", respectively). What's missing?

Well, we know that every complete sentence needs two things: a noun and a verb.

We have a noun (children), but we don't have a verb!

To change this into a complete sentence, add an action for the children to do, such as "walk", "jump", "dance", etc.