1) You cannot all of a sudden provision content differently to a user who has an unapproved device with their preferred accessibility stack and/or hardware.
2) Even if attestation does not involve tracking, effectively forcing children into an ecosystem that tracks them can be deemed unlawful by the FTC. Providers cannot foreclose all means of access to content that are not in a tracking ecosystem, because it violates the rights of children.
The proposal is probably legally negligent because it does not exercise the ordinary standard of care expected of senior technologists. Providing a tool that affects hundreds of million of children and people with disabilities is not a joke.