Which is more correct or what is the difference between them?
The first translates as childhood-onset and the latter as in the early childhood
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Sign up to join this communityIt depends on what you want to express. "onset" means the beginning of a disease or "something unpleasant", so it can be prepended with whatever word you want that express epochs or events ("childhood-onset, early-onset, sudden-onset").
Childhood, by itself, means "infancia"
on the other hand:
For me both "de inicio en la infancia" and "en inicio de la infancia" seem a little bit artificious.
As jonayreyes said, if you're trying to mean that something unpleasant begins "que se origina en la infancia" or "cuyo inicio se da en la infancia" sound more natural.
On the other hand, I'd translate "in the early childhood" as "en la más tierna infancia" if you're in a very formal or lyric context. Otherwise you could also translate it as "en la etapa infantil" or "cuando aún se es muy pequeño" in a more common context.
I'm sure we could help you out much better if you give us a little bit more context!
Cheers!
Ivan