You already have 2 sedans that can transport the family when needed. IMO, I'd either keep the Boxster or get the 911, because as another boarder so accurately put it, "once you drive a Porsche, everything else sucks."
FWIW, I have a 6 month old. At the time, my wife and I only had a Boxster as a weekend fun car, since we lived in NYC and didn't really need a car. Once we knew the baby was coming, we got an 08 A4 as the family truckster, but kept the Boxster, and I got the airbag deactivation bar installed for use with a baby seat.
When my son gets old enough to sit on his own in the back seat, I'll prob. go for a used 997. IMO, if you get another european sports sedan/coupe, it'll feel too similar to your Audi or MB, so what's the point? The Boxster is for fun when you need a little escape, and since you already have ample family haulers, I'd opt to keep the fun in your life.