。
She's careful about her child's nutrition.
Literal
She [topic-は] child [genitive-の] nutrition [target-に] is-paying-attention.
気をつける ('to be careful, pay attention to, watch out for') is one of the most useful set phrases in Japanese — literally 'attach one's spirit/mind' (気を + つける). The thing one is being careful about takes に. The progressive ~ている form 気をつけている marks ongoing, habitual carefulness — not a one-off act of care but an ongoing parental vigilance.