Different ball game (different planet)
I had a BMW 328i Cabriolet, along with the A4 probably the most practical cabs on the market (there's probably a merc. to).
our new 993 cab, like all Porsche is a sports car, not a fast well handling coupe with the roof taken off (no offence the 328 was a great car).
have someone look after the kids and go and test drive a couple. It's the only way to understand the thing. Then have the serious sit down and chat with the other half, who will have at this stage understood that the 993 is a sports car and not anywhere near as "comfortable" and what possible need is there to go that fast around corners.
Then if the answer is still yes take the kids for a fitting. Mine are 6 and nearly 3. We don't go far 4 up, school run mostly. (We have a family car for long journeys).
Our six year old doesn't need a booster in the back as the 993 is only fitted with lap belts and I have made a simple modification to the Porsche Prince child seat so that it fits to my (and my 3 year olds) satisfaction. search for child seat to find my thread and details. The kids have nothing to look at apart from the back of the front seats, but they love the sound

the other alternative is to have the air bag dissabled in the front and use a regular 3 point child seat like the maxi cosi priory.
hope this helps, bit of a ramble