。
She's really a good kid.
Literal
She [topic-は] truly good child is.
実にいい子だ uses the casual copula だ and the everyday いい (good) with 子 (child/kid). Despite 実に being a literary intensifier, the rest of the sentence is casual — creating a mix that sounds like an adult earnestly praising a child. いい子 is the standard expression for a well-behaved or good-natured child.