1. Age them up to young adult using the birthday cakes, then select the sim, choose move, and there will be a checkbox on the right side for kick out.
2. Same as above but enable cheats. Then shift click the sim to trigger age transition, then kick out.
3. Don't take care of the child. The social worker will come get them eventually because of neglect. Easier with infants and toddlers since they can't get around as well as a child. You will have to put of with negative moodlets though.