I'm another vote for don't tell them about this guy yet- not because you're not divorced but simply because I think you need to introduce the whole concept of mom dating slowly and not have it be about any one particular person. Also, I'm not as concerned about the divorce/separation status so much as how soon it is to have settled down to one guy that seems to be serious enough to think you need to have "the talk" with them. I think that would be overwhelming to your kids. When I was divorced, I hid it completely. Only went on dates when my daughter was with her dad for his visitation time. That was the wrong thing to do because when I did become involved with the guy I thought was the one, 7 years after my divorce, she was totally threatened by it and felt completely alienated from me. Who could blame her? I'd been covering what I was doing for 7 years. By this time, her dad had remarried which was okay with her, but the possibility that someone else was the object of my affection was overwhelming to her and it made for her having a rocky relationship with my husband. So, if I had it to do all over again, I would have outings, openly, with friends, whether girlfriends or male acquaintances, on occassion, like maybe every other week as though it were no big deal. (You also still have the visitation time to date) Then she could have had the chance to accept that I was a person with friends including male friends and maybe wouldn't be so threatened by it. I think the talk you need to have with them is just a general high level talk about mom is an adult who needs to socialize with other adults so you might from time to time go out including having "dates" and that this doesn't change anything about being their mom and loving them and they will always come first.
On another note, unsolicited advice from one that has been there, it sounds like you and your ex need to work on coparenting. This could be real difficult if he is a person with NPD, I know, but divorce ( and remember I'm divorced) is harder on kids that we think and harder than what you see on the surface. I way underestimated the impact on my daughter and it's too late for me to change that so I always preach it to others. Invest some time in thinking of ways that you can coparent with the kids, talk to a counselor (if you can get him to go, hard with NPD) about best ways to do that in spite of his affairs and manipulations. In the long run, when they are grown, you won't regret it. My heart goes out to you, it is so hard to raise kids by yourself and always have to be the bigger person. Be strong.