J.M.
I've been in your situation. If your girls know this man and have been around him and his son, there really seems no need to give your dates a title, you just explain that this time, mommy and Mr. "Jack" are going to go out by themselves. Kids are smart and resilient. With my son, I never had an actual conversation about dating, when I finally met someone worth while, and I felt that this was going to be "it", we did things together and then sometimes alone, and my son never asked questions. It just seemed normal to him and part of life. Even when we decided we wanted to get married, we ALL talked about it openly, and when my husband did ask me, he asked my son first. The point I'm trying to make is that I think that if you make it a big deal, then it will become a big deal. Just don't make a fuss. If you do fun things together with all the kids, I'd make it a friend event with no obvious change in your relationship, and if you decide it's the real deal, then include those occasional hugs and kisses. When your girls start asking questions or making comments, then you'll be in a better position to explain how you guys feel about each other and where you think your relationship is going. Anyway, good luck whatever you decide, I hope it works out well for you.
J.