を　ｖｓ　は

from this example sentences

息子を貴方におまかせする and
息子はあなたにおまかせする。

are both translated as "I commit my son to your care"

Why is 息子 marked by を in the first sentence and は in the second sentence and yet meaning is unchanged?

息子 can be interpreted as the subject of お任せする in the second sentence. Although 息子は間違いなくあなたにおまかせする, 息子は多分あなたにおまかせする or 息子はあなたにおまかせするだろう or like that would be more common for the case, since 任せる is other person's volitional action. Even when 息子 is the object as same as the first sentence, those two sentences have different nuance. は can perform as a contrasitive marker (e.g. I commit my son, not my daughter) in the second sentence.