She's the middle child of three.

三个孩子，她是老二。

《牛津词典》

Young Bill, the middle child, was a diligent learner, reading a whole set of encyclopedias.

排在中间的盖茨学习非常勤奋，曾从头到尾读完了一整套大百科全书。

youdao

I was the middle child of three, but there was a gap of five years on either side, and I barely saw my father before I was eight.

我在家中三子女中排第二，其余两位分别比我大五岁和小五岁，而我在八岁前很少见到父亲。

youdao

In some cases the child takes the mother's surname as a middle name and the father's name as the family name.

VOA: special.2009.07.06

Alcatel's One Touch S'Pop is positioned as the middle child within the company's budget lineup.

ENGADGET: Alcatel's affordable OneTouch Pop phone lineup revealed for Q1 arrival

Their oldest had just turned 6, their middle child was turning 5, and their littlest was 3.

CNN: Guiding children through grief and loss

For our middle child, who's six and was particularly close to Sam and idolised him, he's coming to terms with it, with help.

BBC: Fuller family deaths: Aunt's 'sleepless nights'