What adjective I can use to describe "happy childhood" ?
what idioms do native speakers usually use to describe their "happy childhood" ?
I only understand "happy-go-lucky", what else ?
If you give me examples of their usage, it will better.

Question

Answer

I can't think of any. I would use words, rather than idiom.  
I also don't think that 'happy-go-lucky' really applies to childhood, or that it is a synonym for happy. It is more a gentle version of 'reckless', and therefore more applies to teenagers.

집에서 편안하게 언어를 배울 수 있는 기회를 놓치지 마세요. 경험 많은 강사진을 살펴보고 지금 바로 첫 수업을 신청하세요!