Growing up, I always understood my mother and father loved each other and only each other (so far as monogamy went). Why treat poly as something different? Why not just say, hey, kids, these are our friends, they'll watch you when Mommy or Daddy can't (assuming your kids are little), they'll come with us to the park, to family gatherings? As the kids grow older, they'll understand what those "friends" are, and if they don't, and they care, they'll ask questions, which should be easy enough to answer.
If they're older, well, you can just take them aside and explain the structure to them, simply, as in, Teresa is part of our family, we love her very much, etc. etc.
They won't care about your sex lives, just understanding the role they'll play in their lives (well, maybe they will care about the sex part too, but in that case, well, no advice