children's body is developing???

[Lily of the valley]

I'm looking for a phrase but don't have a clue how to put it. I want to say that "Children need to sleep a lot because their body is developing". I'm a bit lost because I haven't been able to to find the equivalent of either of the words. Instead of the word 'body', I was thinking of something like 'organism' but I found that it wasn't used in this context.

[SoothingDave]

"their bodies are developing"

[Lily of the valley]

"their bodies are developing"

Apart from the plural, is it correct to say?

[allenman]

Your original sentence is correct except for the plural issue. I know of no other way to describe it more correctly.

Not a teacher -- AmE native