Some of it is families in the gap where they can't get employment that pays fully for family healthcare, but make too much to qualify for CHIP/Medicaid.
Some will have been disqualified for other reasons, perhaps.
Do we know what fraction? If most American children have access to healthcare–a claim I'm sceptical of–then I'm not seeing an urgent problem that has to be solved by the healthcare system.
Everyone in the USA has access to emergency care and stabilization, the practical results of which are that for emergencies, you get helped, payment is worked out later.
If you have no problem with having no intention of paying, the system absorbs the cost.
No they don't, they just don't address it. Neither does your comment.