M.L.
It's much, much better that they hear it from you and your husband than from anyone else - including Grandma and Grandpa.
Undoubtedly your children already know something about divorce from being around friends and neighbors. You'll just have to tell them straight out that - like Billy's parents and Jenny's parents and the neighbors on the next block - they have decided to divorce and stop being married. (Since they are "already into other people," as you write, it seems divorce is more than just a possibility.)
Anticipate their questions. In fact, encourage them to ask questions - questions not only when you tell them but questions any time down the road. This could be a good teaching opportunity. (Be aware that one question might be, "Are you and Daddy going to get a divorce?")
If they happen to say, "Why didn't you tell us about this when it started?" you can say, "This whole thing has made me so sad and confused that it has taken me a while to be able to talk about it, to you or to anybody. It can be hard to talk about a thing that makes a person so sad." That's something they need to learn anyway.
They may look to you all for a lead in what to say to Grandma and to Grandpa when they're with them. What will you answer?