is easier, than you might think.
There are a number of ways it can be done, but here's how I did it:
Confederate Partition (succession law) means that all children will inherit equally. If you hold 10 (potential) Kingdom Titles, these will be split between your ten children/sons (1 primary heir + 9). This is very achievable within one lifetime if you create a custom character, make him 16-20 years old, and start aggressively conquering. While every ruler works, I recommend a ruler with a faith that allows concubines or secondary spouses, otherwise you'll never get enough children within one lifetime. However, if you die, that's not a problem. Just keep capturing kingdoms and let Confederate Partition do its thing, it will merely delay the inevitable.
EDIT: DO NOT CREATE AN EMPIRE. EMPIRE WILL KEEP THE KINGDOMS TOGETHER.
I did it with the Tribal Kingdom of Ghana
in Africa (Maghan Zoumana of Ghana); it's surrounded by a lot of weak rulers and enough kingdoms to spare. You'll never have a neighbor strong enough to challenge you.