。
You're going to get lost, you know.
Literal
Lost-child [to-に] become-[completely-してしまう] [emphatic-よ].
迷子になる ('to become a lost child / to get lost') + してしまう for the sense of 'end up doing, do regrettably / involuntarily' — here warning someone that if they don't stick close, they'll inadvertently become lost. The よ marks the warning as new information. Parent-to-child admonishment.