It really depends on what makes you happy about owning a 911. If you're content in the fact that you own an iconic car, and a turbo at that, which you can use as and when you chose then no need to sell.
However, if you're someone that needs to feel you are using your car to at least some of it's potential then I understand you would think about selling.
I think the down fall of the 996 is that they are just so competent, so just using them for short trips etc is never going to feel like you are properly using it. I agree with Ghianightmae, it's not the total miles you do, it's the quality of those miles.
I've owned a 996.1 3.4 for 3yrs now and I also own a 1970 Alfa 1750gtv. I do about 1500 miles in each a year. I've done some decent trips in the 996, done a few track days and generally enjoy owning it. But I always feel like I should be driving it to the south of France or the 'Ring to get the most out of it, which like you I find hard with 2 small children in tow. Where as every trip, even just starting up the think, in the Alfa is a proper event.
As an all round car enthusiast though, I have to say that a Caterham itch is an itch worth scratching. I've had 2 and in my experience there really is nothing else offering the same direct, raw, light weight feel.
My last one was an ex academy car, and while I did use it on the road, it was mostly used on track days and for 3 yrs of competing in the club sprint and hill climb championship. While the mileage was probably only a thousand a year, I was very content knowing that 99% of those miles were doing exactly what the car was intended for and so I felt I was using it to it's full potential. So I do agree, if used properly, a Caterham is a good shout to get your thrills for some quality miles, and then shove back in the garage until the next time.