Health insurance isn't free either and it's way more expensive than Medicare. We're all also already paying for Medicare...

GP is pointing out that Medicare costs for the individual are about the same as ACA or cobra. It's not cheaper, unless you have the work credits to cut down on part A premiums.

So Medicare as-is for kids wouldn't be significantly cheaper than ACA for kids.

To make it cheaper, you'd need to either substantially increase the subsidy on Medicare, or decrease US medical costs (administration costs, drug costs, doctor salaries, etc)

Commercial health insurance is more expensive than Medicare because original Medicare Part A/B doesn't cover prescription drugs, and because commercial plans effectively provide a hidden cross subsidy to Medicare. Under a "Medicare for All" scheme, people would still need to purchase prescription drug coverage (Part D). And because Medicare reimburses providers at very low rates (often below their costs) they have to make up the difference by charging commercial health plans more.