Q: Family sharing - how to create a separate account?

My son (4) has his own ipad, but I've always used my Apple ID to purchase apps for him. I'm trying to set up family sharing. Recently I read on Apple's website:

Kids under 13 can have an account too.

Now kids under 13 can have their own Apple IDs. As a parent or legal guardian, the family organiser creates the child’s Apple ID and adds the child to the family group. Ask to Buy is turned on by default, but the organiser can also limit the content kids have access to on their devices through Restrictions on an iOS device or parental controls in OS X and iTunes.

How can I set this up for him. As I've tried creating an Apple ID for him but I get this message You are not eligible to create an account at this time.

I'm assuming because he is under 13.

When I do eventually create an account for him, will I be able to share previously purchased apps that I have purchased before setting up his account?