は子供の扱いがうまい

is good with children

→ は
phrase Rare #39,550
is good with children The は marks the person being described. 子供の扱いがうまい literally means being skilled at handling or dealing with children.
JA: 姉は子供の扱いがうまい。
EN: My older sister is good with children.

トムは子供の扱いがうまいんでしょ

Tom is good with kids, right?

彼女は子供の扱いがうまい

She’s good with children.

トムは子供の扱いがうまいのよ

Tom is good with children.

彼女は子供の扱いがうまいんだ

She's good with children.