SSO for Master AND automatically, SUB Accounts

[ZumKuckuck]

As Owner and Super Admin I implemented SSO on the Master account and this is working well.
Actually I was expecting, this automatically gets replicated to the 5 Sub Accounts that I have
but, no, it does not.
Question here is: how do I activate my SSO to become available/mandatory for my Sub Accounts?
I can't believe I need to setup a Vanity URL for each of those and do again the manual SSO related work
Thanks for helpful inputs