Welcome, Kernow! I'm Leelee, and I'm very new on this forum, too.
Your situation sounds amazing, and that it works very well for everyone involved. I'm a single (divorced) woman with children who was briefly part of a not-very-functional "v" arrangement (I was the new partner to the male hinge). While that particular relationship didn't work well, I know that it's something I want to find again.
If you stay with us, I'll be interested in hearing more about your story, especially about how your kids feel about it. My kids are teens too, and I'm not sure how they'd react to a poly relationship, or if I'd decide to tell them or not. "This is my boyfriend and this is our mutual friend" seems easy to get away with... but also diminishes her role...
Hope you find the support and friendship you're looking for here.
Leelee, all alone.