How many situps should a boy at age ten do to get a sixpack?

There is no "should do" amount.

You are recommended by health officials to do as many as you can physically achieve, then try and beat that record the next time you try sit-ups. Start small and work your way to a higher number. If you strain yourself too hard, you could end up damaging yourself, so,onlydo as much as you feel you can achieve per session.