We were together 23 years, with 3 kids, never married. My DD was Christmas and then my birthday Feb 2. He had been talking about counseling, and told me he still loved me.
When I confronted him about a photo I found of him in a bar with a different woman on my birthday he then declared he'd been dating her for a week. He flew to Vegas without a word to me and married her only knowing her for two weeks.
He bought her a flashy EXPENSIVE diamond ring, a trip to Vegas and I found out he married her on her birthday 8 days after mine. I found out when the registrar called my house to ask about "my" wedding.
He has not spoken to me but once since then. His stuff is still in the house. Our kids are 16-21 and he is pushing his new wife on the youngest. This woman has been married more than 4 time and has a child that doesn't live with her across the US. Our daughter asked, and he just kept telling her "it's complicated" when she asked if she'd been married more than 4 times. She doesn't want to spend time with her, but he won't remove her from the situation to allow them to try to cope with this new arrangement.
I've disarmed the Narcissist by only emailing him telling him his new wife is not my concern, except when it affects our children, and that I want no contact other than email until she reaches 18. I'm sure he intended to try to force me to come in contact with her, since this entire situation has been designed to cause as much pain as possible to me. He left me without a word, with no remorse, and moved immediately out of a 23 year relationship where he never married me to married in 12 days. He has done all the things he talked about doing with me for her. It was a direct response to being called out for his infidelity. He could have left like a man, with dignity and respect, but instead he slithered away like a coward.
In answer to who marries someone that hasn't met their kids, I would have to say a person that is selfish, and Narcissistic. Your children are an important part of life, and to subject them to a sudden change in family dynamic isn't fair to them. They need stable parents, that respect each other, and have a common goal of the best interests of their children. Kids need time to adjust and I wouldn't introduce a child to a "love interest" in any aspect until they've had time to meet them, and build a bond, over at least 6 months. I'd never introduce them until the relationship was headed to a more serious commitment.
I would suggest you try to speak to your children. They might not be able to grasp what has happened, or need time to process the situation. Let them know that you can't control what other people do, only how you choose to respond to it. I understand your pain. I'm still trying to wrap my head around this entire thing, while also having to provide emotional support to children that are struggling with coping.
The pain is real, but once it's gone, I will be stronger and more alive than I've been in years. She's done me a favor, she's freed me from years of verbal abuse. I will no longer have to look over my shoulder, or second guess where he is or what he's lying about now. He's ALL hers now, and although I can't see the bright side yet, it will get easier over time. I WILL be happy again, but for now I have to love myself, and be content to heal and grow.