。
She's good with kids.
Literal
She [topic-は] children [object-を] handle [nominalizer-の] [subject-が] is-skillful.
扱う ('handle, deal with, manage') is a remarkably flexible verb — used for operating tools, managing relationships, dealing with customers, or, as here, knowing how to relate to children. The pattern [verb dictionary]+のがうまい uses the nominalizer の to turn the verb phrase into a noun, then が to mark it as the subject of the skill predicate. うまい is a colloquial cousin of 上手, with the side meaning 'tasty' that context disambiguates instantly.