Nate Tempest

+1 on clicktight seats; the mechanism works well for the raised belt positions in the back of a 911. Mine is a 997, with less space than your 991, and I also have a 5 and a 3 year old. Currently using two seats - a harmony booster and a Britax Boulevard clicktight. In theory the Boulevard seat will last long enough to get the younger kid into another booster.

In my wife's car, both kids use the Britax Frontier linked above, with 5 point harnesses. http://www.toysrus.ca/product/index....uctId=55114336 It's too big for the 997, but if it fits in your 991, it would definitely fit the kids.