tell on someone

(idiom) to tell a parent, babysitter, teacher (etc.) about the wrong actions of another person.

Example sentence:

You ate the cookies, I’m telling on you!

Stop hitting me or I’m telling mom on you.

Note:

tell on someone is usually used by children only.

Similar idioms:

1. rat
2. catch on
3. toss one’s cookies
4. see through someone
5. keep someone in line
6. level-headed
7. stop by
8. stop in
9. bark up the wrong tree