Because you caused this child to get seriously hurt, all their summer vacation plans fell through, you know?
Literal
You [by-に] serious-injury [object-を] was-made-to-suffer [because-せいで], this child [possessive-の] summer-vacation [possessive-の] plans [topic-は] all came-to-nothing become-[regret-てしまった] [explanatory-んです] [assertive-よ]?
The causative-passive させられた is accusatory — 'you made this child suffer a serious injury.' おじゃんになる is colloquial for 'to come to nothing, to fall through.' The final んですよ？ is confrontational: explanatory の with assertive よ forms a rhetorical 'don't you realize?' rather than a genuine question. 貴女 (あなた with the 'noble woman' kanji) is used with barely concealed anger — あなた in confrontational contexts signals cold distance.