I recently told mine, in a general way. I had been S just over a year, and SO and I have been dating, well, it depends on how you count, but 8 mos. or so? (Yes, it was fast. Consider it one of those relationships I didn't post about because I didn't want the 2x4s, as mentioned in the other thread. lol)
Anyway, it was at the point where I do not want the kids to have significant contact with him, but a once-in-awhile thing would be good. I was thinking they might guess anyway, because they ask what I did and who with when they are away, and I wanted to just have it out. It felt dirty to hide it. (They had met him once before, when we were not together.) I talked to the X about it, as a heads up, and we discussed how to approach them with a general dating discussion based on each of the kids' particular temperaments and ages.
Anyway, my middle DD, 10, brought up dating in a general way, and I used it as opening to discuss the idea of me dating in general. She promptly hid under a pillow and would only squeak at me for awhile. Eventually it came out that she was thinking "and get married, like, tomorrow" and once we cleared that up she still didn't love the idea but was more accepting. She has not wanted to discuss it further but has stopped having some of the symptoms she was having of anxiety etc and has graduated from IC (to whom she never mentioned the dating thing).
DS, 12, it came up in conversation also. We were discussing some work SO and I had done around the house, and DS asked me if he had done that kind of work for a living, so I explained that he does but that he was in school with me in my program as well. I made a joke "see he's smart and handy, you need to aspire to that" or something like it, and DS looked at me with a grin, gave me finger guns, and said "and he's available!" Later I asked him about the comment and he said he was just joking around. I said, well, how do you feel about the idea of me dating? and he shrugged and said he figured I would at some point. I asked if he felt funny about it, since he was going to be dating in a few years and I sure felt funny about that! He laughed and said it was fine. I asked him how he would feel if I told him that actually I was dating SO. He shrugged again and said "he's nice". It was no big deal to him.
The 6 year old I havent said anything to. I doubt she would know what it meant or really care.
DD's therapist has suggested, now that I have introduced this idea, that I have SO take me on a date in front of them, to make it concrete. Like hire a sitter, let them see me get picked up etc. That is so not how our dating relationship works, though, and I am not inclined to get a sitter on my kid time, so I don't think so. I'd rather have occasional outings or having SO here for a meal so they can get to know each other a bit, just so that if things do progress in a more "serious" direction hey will not be strangers to each other.
For me waiting this long to introduce even the idea of dating has been a good thing. The kids needed time to adjust to the change in the family, I wanted to be sure this was not just a fling, and since the relationship is deliberately moving slowly I want this to be a slow process too. I talked to both kids about how dating is a process where adults get to know each other, and that sometimes that means one or two dates and sometimes that means longer, but that it takes a long time to really know someone so they dont need to worry about my bringing anyone into the family for a long time if ever. They are of course mostly concerned about how this will effect them so I think they both wanted to hear that it wouldnt for a good long while. I am mostly going by my gut read of where the kids are and where the relationship is. Its a funny in between place right now, where I feel like they need to know each other a little for things to progress with SO but that I want that to be an equally slow getting to know you process. SO has no kids and is letting me take the lead on decisions with regard to them, though he is not interested in taking on a role with them at this point either so there is not that pressure. Its just a balancing act that I think I am doing okay at so far given all the factors in my situation.
Hope that helps!