The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

That's a bit Childish, isn't it?
The Big Bang Theory Season 10, Episode 11

Don't make it sound CHILDISH.
The Big Bang Theory Season 12, Episode 14

- You're being CHILDISH.
- No. He is.
The Big Bang Theory Season 8, Episode 21

Do you know how CHILDISH you sound
right now?
The Big Bang Theory Season 3, Episode 7

It's a CHILDISH trick designed to confuse
and intrigue simpletons.
The Big Bang Theory Season 4, Episode 18