baby vs sweetie

baby and sweetie both are nouns.

Word	Noun	Adjective	Verb	Adverb
baby	Yes	No	Yes	No
sweetie	Yes	No	No	No

As nouns, baby and sweetie are synonyms defined as:

Other synonyms of baby include beloved, darling, dear, dearest, dearie, deary, honey, love, sweetheart.

baby (noun)	sweetie (noun)
a project of personal concern to someone	a person loved by another person
a very young mammal	a beloved person; used as terms of endearment
a very young child (birth to 1 year) who has not yet begun to walk or talk
the youngest member of a group (not necessarily young)
an immature childish person
an unborn child; a human fetus
a beloved person; used as terms of endearment

baby (verb)	sweetie (verb)
treat with excessive indulgence