I used to say, when I was a car-mad teenager, I would only ever have enough kids to fit in the backseat of a Porsche: two very small ones.
Never mind that before the Boxster--a car designed to separate junior I-bankers from their bonuses--the typical Porsche owner was more likely to have just traded the mother of his children in for a newer model (or model wannabe).
Nowadays, Porsche is actively pursuing the family man market. Not only did they launch the Cayenne 4-door SUV, they offer car seats for the 911, Carrera, and even for the 2-seat Boxster (mom's on her own, I guess).
Some models can be wedged into the backseats, but when installed in the front, they require a dealer-installed airbag disabler switch.
Porsche car seats are "developed in cooperation with" Britax-Romer, which basically means they're standard Britax with Porsche upholstery and the Porsche logo emblazoned on the front. Another difference is price: at $175-230 for the Britax and $380-410 for the Porsche, that logo works out to around $200. Enjoy.
Of course, this is the same strategy as the Cayenne, which, unless you get the Turbo, is just a VW Touareg with a $40,000 pimp kit (or a year of Ivy League tuition) grafted onto it.