Most children don't have enough credits to qualify for Medicare.

OTOH, very few children have enough individual income to be disqualified from Medicaid, but it's based on household income.

My handwavey plan for universal federalized healthcare includes using the child's income as a qualifier for Medicaid, phased in so the system will hopefully adjust over time rather than get overloaded to collapse. Also reduce the Medicare eligibility age over time. A solution that takes decades to roll out leaves a lot of unsolved problems, but adding a large number of people to an existing program in one fell swoop feels like it's going to be a negative too.