tattle on (one)
Especially of children, to inform an authority figure of one's bad behavior. I can't believe you tattled on me like that to mom and dad—I'm never telling you anything ever again! I know you think you're trying to do the right thing, but you're going to have a hard time making friends if you tattle on your classmates for every little thing they do wrong.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
To inform that someone has misbehaved: My brother tattled on me for spilling the glue.
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.