The thing many of these have in common is that connections help you. In other words, if your parent is in x career, it means that you get a starting bonus if you do the same. On top of this, most children have some knowledge of the job conditions: Familiarity seems to lessen risk (even if it doesn't - it is the reason showers seem safe but airplanes might not).

Legislators seem to get more done if they are well-connected, for example, and a parent introducing you to their connections makes you a better legislator, probably at a younger age than your parent. Yes, you can do some job shadowing at this other dentist's office: I can put in a good word for you at the electrician's union so you can get into the schooling they offer. Yes, you can work at my plumbing company and learn - it isn't like this state requires licenses, so you'll be just fine.