I Don't have kids, but if or when I do, I would tell them, because any partner of ours is a potential step-parent to them, and I feel they deserve to know something that has that much impact on their lives. In some way, a live-in partner has more impact on children than on their metamour, simply because children spend more time in the house in every household I've seen (school usually lasts less than work, and parents can come and go as they please will kids need permission to go out, and depending on their age, an adult along with them).
Mind you the kids in question are oldish and would have more freedom, and nothing in the posts implies that the relaitonship is primary or is going to become so, unless I've mis-read.
Still, I can't see myself lying to my kids about something like that. I don't have anything to hide, and every time I present a partner as a friend, it is a lie. Get them used to it early enough and they'll think nothing of it.
This isn't very pertinent to the OP, but it was in response of whether to tell children. I personally don't care about my parents' lives much, but I don't think I would have enjoyed living a lie either. To a kid, their parents' relationship is important, you see it when divorces occur. Being lied about them for years... I don't think a lot of them would be happy about that. It could make one wonder if anything from their childhood was actually true, up to and including their parents' love.