“That one's not appropriate for children.”

You're at a video rental store with your children. You told them they could choose a video to rent, but they picked out a violent horror movie. You want to tell them that they can't watch the movie. And you want to say it in a way that sounds adult, so that your children don't think that you're talking down to them. You say:

That one's not appropriate for children.

not appropriate for (something)

When you say that something is "not appropriate", it means that it's bad. But the word "bad" means that something isn't good in any situation, while "inappropriate" means "bad" for a specific situation. You tell what situation you're talking about with "for ___". For example:

Did you see what she's wearing? That's really not appropriate for work.

You can also follow "appropriate" with "to (do)":

That's a good song, but it's not appropriate to play at a wedding.

 The word "appropriate" is intellectual- and adult-sounding.