I have mixed feelings about this. I was not able to be poly and out before kids because I didn't even know about poly until after I had a kid.
Right now I'm in a LDR with TGIB. As long as it stays a LDR, neither he nor I see any reason to be "out" to family (our parents or our kids) about it. We're out to friends, but to family we're "best friends" (which is true, just not the whole truth!) and TGIB gets included in things as a best friend/family friend. My kids are too young at this point to know anything about sex, and I'm okay sticking with "Who I sleep with is none of their business" in regards to both my kids and the rest of my family. While it goes against the grain to not be fully truthful and honest, the cons of being "out" at the moment outweigh the pros.
Eventually TGIB hopes to move here, and once we've managed that chaos and things have settled down, I'll tell my parents about my relationship with TGIB. My mom will likely be upset and fear the damage it will do to my marriage and my kids, but by then I'll have years of "Well it hasn't so far, so the odds are slim" to back me up. Then, and only then, will I tell my kids, precisely because I WON'T put them in a position to have to "not tell" or even lie about it. I also don't think my kids will much care, because even now they're used to TGIB being a part of our lives, and they still won't need to know what goes on in my bedroom!
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack