Kevin, I wouldn't be purposefully dishonest with the kids - even if it is to surprise them. I think it'd be OK to be mysterious, I understand what you're wanting.
My W's A was a little different from the norm - OM was seriously a complete dork, and absolutely opposite of W. He simply was very quiet/meek, and very nice - heh, calling someone "very nice" is what you usually say when you break up with them!
Also, he lives 2 hours away, and has three kids, and they aren't real well-behaved. Also, as far as I know the A wasn't a PA, or if it was the PA part was very short-lived. It started out in mid-December, and they stopped really "dating" in mid-February, so even then it wasn't hot and heavy.
And, if 6 months is an average, you'd have to know what the extremes are. I would suggest a range of 1 month to 12 months - but who knows where you fall?
