。
She has ten children, you know.
Literal
She [topic-は] ten-people [of-の] child-having [explanatory-なんだ].
子持ち (こもち) is a compact noun meaning 'having children' — 子 (child) + 持ち (having/holding). 十人の子持ち = 'having ten children.' The なんだ ending (casual form of のです) marks this as explanatory or emphatic — the speaker is sharing a surprising fact. The information feels like gossip or an aside: 'can you believe it, she has ten kids!'