Because of their age children have no chance to earn thier own money in most countries. I'm not saying children should work all day long, no, they should rather study, but it they have a chance to earn a little, working not more than 1 hour per day, that's great IMO. And Bitcoin gives them that chance.
Bitcoin gives the chance of earning to all the age group. Even children can benefit through the faucets and captcha solving which are simple. Importantly as said above children should never be forced. We can teach them, based on their own interest let them work on it. Even working a single hour gets a good amount in satoshi.