It's a famous Scientology doctrine. People used to believe that children are not as capable as adults and therefore we should act differently and more carefully around them and look out for them. The Scientology doctrine is that they are "adults in small bodies" and so whatever they do is punishable as if an adult did it.
I don't think that tracks. Surely if the spread of that doctrine were the reason, we'd see children having more autonomy and a recognition that it's immoral to e.g. take their property, or confine them against their will.