early childhood vs infancy

early childhood and infancy both are nouns.

Word	Noun	Adjective	Verb	Adverb
early childhood	Yes	No	No	No
infancy	Yes	No	No	No

As nouns, early childhood and infancy are synonyms defined as:

Other synonyms of early childhood include babyhood.

early childhood (noun)	infancy (noun)
the early stage of growth or development	the earliest state of immaturity
	the early stage of growth or development