I think OP will have to walk the fine line in the age-old debate: the paradigm of telling children what to do, and the paradigm of letting them figure it out by themselves.
I think many kids enter the tougher professions due to a combination of both. I do not completely believe a kid who passionately wants to become a surgeon. Where is it coming from? How do you develop a passion even before your first serious Biology course?
It is a combination of ability, some interest, parental influence, and social norms.