I don't think that 12 year olds are ready to chose their own careers. They aren't ready because they most likely just started middle school and didn't fully mature yet, and lastly because if kids cant even handle homework and dislike getting up early to go to school, theres no chance of them getting up to go to work.
Since kids didn't fully mature, they won't take their work seriously. They might skip a day or won't finish their work , not realizing that their boss will fire them if it continues. This could cause much kayos in our world. Also since kids don't want to go to school or get up in the morning to do so, I am sure they wouldn't want to do that for work. This proves kids aren't ready to choose their careers.
Even though I don't think kids are ready to choose their careers, but if I had to choose my life assignment now I would choose to be a nurturer. I would choose to be a nurturer because I enjoy playing with little kids and taking car of them.