I think you're wrong about children not having purchasing power. Remember all those toy commercials on TV during Saturday morning cartoons? Those advertisers weren't just throwing money away, they were marketing to kids, and those kids then bugged their parents to purchase the toys.

The same holds true for YouTube; in fact I might go so far as to say it's even more powerful today. Kids are choosing the videos to watch and people to subscribe to. That means they have an interest and are actively participating in the process. I would think this would make them a better target with a higher ROI. Especially if the investment is a couple of hours of video creation.