。
If your parents find out, it'll be trouble.
Literal
Parents [to-に] becomes-known [conditional-たら] troublesome matter [result-になる] [emphasis-よ].
知れたら (if it becomes known) is the conditional of 知れる — the intransitive/potential form of 知る. 面倒なことになる (it'll turn into something troublesome) uses こと as a general situation noun. よ adds warning emphasis. A very natural spoken warning.