How to transfer game progress from Parents account to an associated Child account.

No, it's not possible to transfer game progress from one account to another. Your child will have to start the games again on his account.

game data is saved to the individual profile.

if started with a new profile a new game start would be required.

You can create a child account on your current parental account. You can not transfer games, saved progress etc. You can set the parental account to Home Xbox and the child account would have access to the games but not the saved progress.

It is not Microsofts fault that a child account was not set up.

Unfortunately you can't change an adult account into a child account and you can transfer items among different accounts. Just make a new child account for your child and associate your account as parental account.

You can't transfer licenses or content from one account to another.

The accounts Parent / Child are separate accounts. The parent account just manages the child account.

The child account will remain a child account until the birth date that was used for that account reaches the age of 18.