There's every other kind of family type, pretty much, and polyamorous families have some unique joys and challenges that it would be nice to share and discuss.
Actually, I just had another look, and there are clearly some family types, including my own, that are not included. My family type will have a larger representation in the population than yours, I can assure you, since its population is increasing by over 100,000 population per year in the US alone.
If I needed information or discussion in this regard, I'd either post elsewhere on this site, or use a specialist forum.
I just googled for poly forums. There's quite a few out there. Great sources of info and support if that's what you want. Which makes me even more sure your post is not a genuine desire for a useful discussion board, it's a desire to discuss yourself and your new set up.
And if I needed confirmation of that, I'd look to the irony of your weekend posting. While initially suggesting you wanted a place to discuss the unique challenges of your family, out of 105 posts, you then proceed to discuss mainly yourself, your DP, and his OSO, with barely a nod to your own small children. That's an awful lot of time spent on you and your relationship rather than your actual family.
Why not just post in Relationships, as it seems that is really what you want to discuss?
If every fringe group wanted their own subforum the site would be an unmanageable and barely navigable mass of barely visited tumbleweed sub- sub- sub-forums.