I don't think it's a market failure. The reality that password-gating software installation at the OS level can be done on most desktops but not most phones is the opposite sign of a market failure. Mobile OSes have increasingly stripped down capabilities in recent years precisely because of anti-competitive practices. The reason standard desktops have not evolved even better parental control features is not because they're not doing better than phones under a free market. They are already doing better in spite of the fact that most kids use desktops a lot less than they use phones. It's just that the absolute level of demand for parental control features has been low until recent years, and even this recent wave of demand is somewhat manufactured.