。
Her major is child psychology.
Literal
She [topic-は] child-psychology major [is-だ].
Three Sino-Japanese compounds stack into a single subject area: 児童 ('children, juvenile') + 心理学 ('psychology') = 'child psychology'; with 専攻 ('major, specialty') the phrase becomes 'child psychology major.' Japanese forms long technical terms by chaining 漢字 nouns without particles or の — the result is dense and information-rich. 専攻 is the standard word for an academic major or specialty; the verb form 専攻する means 'to major in.' Note that this is a copula-final sentence: 'X major + is.'