。
She's gentle with children.
Literal
She [topic-は] children [topic/contrast-には] gentle is.
には is a compound particle: dative に ('to') + topic は ('as for'). The combination foregrounds 子供達 as the topical context — 'when it comes to children, she's gentle.' Often には also carries an implicit contrast: 'she's gentle to *children* [though she's harsher to others].' やさしい is a beautifully polysemous adjective covering both 'gentle / kind' (toward people) and 'easy' (of tasks) — context decides which sense applies; here it's clearly 'gentle.'