In both of your examples, the phrase that follows describes the story.
If 'story' is the subject of the sentence, you could add a verb to say what the story does, or you could use a form of 'be' to connect the story to the description. That would be the verb.
It is also possible to make someone or something else do something to the story. 'Story' will be the object of the verb, the thing that the verb happens to or changes.
You can make it a sentence by beginning with "This is a story that teaches children the great things you can discover when you limit your screen time on social media." Or you can leave it as a noun phrase, if you wish; this will work if you are using it as a title or subtitle, for example. Your use of the phrase "limit your screen time," which is what I suspect you were actually asking about, is good idiomatic English.