I truly hope that you are writing these with a time lapse. And that you have a fucking good plan to protect yourself AND your children. Because if this is true, it's only a matter of time before someone you know reads this OR someone does just the right Google search.
I coach women co-parenting with a narcissist. I know you are in a tough spot, money or no money. It's not as easy as it looks from the outside.
It's not your fault. That's the main thing I want you to know. All you can do at this point is to act, not react. He is who he is. You were young. Now you know better. I hope to God that PI isn't going to go to the main money man for even more money and sell you out.
Keep your important documents ON you. And your most salable jewelry. If this turns dangerously ugly, you need a quick way out.
I hope this has a happier ending than beginning. I also wish you had someone to talk to besides your PI.