Opinion:
Cheese
I think this is more a brand awareness, advocacy, and corporate social responsibility project.
This will be relatively low cost, and will introduce children to the YouTube / Google brand. It will also create a halo affect for other Google branded services in the eyes of parents.
It will end up increasing income from other services and improve Google’s brand perception. I hope they expand child friendly services into many more of their offerings, and I believe all major tech companies have a responsibility to focus heavily on online safety.