What do you mean by "add a partner with us"? Do you mean you want to date others, or your husband wants to date others, or both? You don't have to share one partner to be polyamorous. You might be imagining ultimately moving one woman into your household, to live with you, along with your kids. Please realize that, contrary to mainstream understanding, doing a "triad" like this is uncommon in polyamory and unlikely to work well, or last. You might go through years of trying to find a woman to "share," only to strike out over and over. And if you introduce prospective dating partners to your young children, your in-laws will have that ammo to use.
Or maybe you and husband already have a woman in mind. Do you? How do you imagine incorporating her into your household to cause the least upset, confusion and danger?
You two could practice polyamory by dating separately outside the home, and thereby not expose your children to your dating practices at all.
Before hiring a lawyer, I recommend reading the book Opening Up for a lot of good and useful background info about how to do open relationships or polyamory successfully.