We three simply tell our daughters (ages four and six) that one of us is going to see [the lover's name], and will be back tomorrow. When any of the two of us go out (or stay in) we just say that we are having some special time together. The kids understand "special time" because we have one-on-one time with each of them as well, to go to the zoo, or the game store, out to lunch, or some other kid-friendly activity.
When they get older, we will fill them in a bit more, but in my opinion it's just not necessary to hide anything, nor indulge in full disclosure. When they start asking questions about it, I expect we'll answer them truthfully, but discreetly. I was never all that interested in my own parents' sex life.