Kids tend to adjust to changes pretty quickly, and I think kids nowadays recognize and understand a far wider range of sexualities and romantic arrangements than when I was growing up.
Before she had been officially told about my situation, Country opened a discussion with me about polyamory and how she couldn't understand why anyone would care if three or more people are in a relationship as long as everyone agrees to it. That made it easier, a few weeks later, for me to come out to her. At that point, her only response was, "Yeah, so? If my stepdad is okay with it, that's what matters." I assured her Hubby's completely fine with it, and she said, "Okay, then do what you do."
I had held off in telling her in large part because I was concerned about her judgment, and also concerned that during one of our conflicts, she might tell her father--who would not have responded well, particularly since he constantly accused me of cheating during our marriage. (I never cheated on him. Not even close.) But her reaction was pretty anticlimactic.
Point being... kids understand and accept a lot more than adults give them credit for sometimes. At your kids' ages, I would almost guarantee they have friends who identify as a variety of sexual orientations, romantic orientations, and gender identities; among my kids' couple dozen or so friends, there is only one girl who identifies as completely straight, monoamorous, and cis-gender. (And that girl is not one of my kids...)
I would strongly advise, though, that you be open and honest from the start. I see others in the thread telling you to wait until there's definitely something going on with you and your friend, and that is valid advice. But *personally*, I would at the very least sit down with your kids (the ones who live with you, anyway) and your husband and say something like, "We want you to know Friend is going to be around a lot more. We consider him someone very close to us, and no one is going to be doing anything behind anyone's back. We just wanted you to know about this so you won't worry, and if you have any questions, please come to us." Having their stepdad be part of the discussion is important, because that will prove to them more definitively than anything you could say that he knows and is on board with this.
Even before I identified as poly, when Hubby and I first opened the marriage, we told Alt and Country that we'd decided it was okay if one or the other of us occasionally went out with a friend. We didn't mention dating, sex, or anything like that; but because my kids witnessed the destruction of my marriage to their father, and particularly because their father told them numerous times that I'd cheated on him, we wanted to make sure they knew up front that Hubby and I had agreed and were being honest and open with each other so they wouldn't worry about our marriage falling apart. In addition to accepting and understanding more than adults give them credit for, they notice a lot more, too.
I just want to add, not to argue but because it seems to be a factor in this thread... religious or spiritual beliefs don't *necessarily* equal judgment or lack thereof. I came out to the pastor of my mother-in-law's church a couple months ago because I used to enjoy attending but had stopped when I started dating Guy. I couldn't stomach going to church knowing that everyone believed me to be the good little monogamous wife and mom, when I no longer was. I didn't want to broadcast to the entire church--especially since Hubby's one hard limit for all this is that his family NEVER find out--but I was considering going back to church there, so I wanted to get a sense of what the pastor thought of polyamory. I told him the situation (at the time, S2 and I were still together), and he said, "Well, there was definitely a whole lot of polygamy going on in the Bible, so I guess in some ways the Bible supports polyamory. You're completely welcome here." Whereas the most negative reaction I got came from a Wiccan friend (no longer a friend, because of this reaction) who said I was "appalling and wrong."