Market is two companies who do not compete in this area at all, because Google literally earns on monetizing attention and Apple since Jobs era uses children as part of the strategy to lock you down in their ecosystem (see emails they had to make available to the court). There are zero serious disruptors and chance they'll appear gets smaller, because of push for device attestation being required for more and more apps.

Making children do an hour of Duolingo before they access open internet is hardly the goal. It's more about limiting their exposure to brain rot content. Existing tool would require you to block it domain-by-domain.

Honestly I can't see less invasive solution for that tool to work than page broadcasting age-rating with http response and device being aware it's owned by minor and refusing to display it.