i have two kids. a newborn, and a 2 and a half year old.
fact: my older boys forward facing britax roundabout seat fits just fine in the
MCS.
fact: the rear facing infant seat also fits, though yes, with the handle all the way down, its a bit tight. its a graco, not exactly sure of the model.
honestly: you are talking about the smallest car on the road. does it have a lot of usable space for such a small car? sure. does that mean its a great only car for a family? depends. but saying it 'failed' is like saying your 2 gallon bucket 'failed' to hold 3 gallons of water.
some things to think about, from someone with experience:
* two doors don't make it any easier getting in and out of the back seat, be it an adult or a child or infant carrier. being low to the ground doesn't help the situation.
* the MINI only seats four, which means the the middle, which i'm pretty sure is the ideal spot for the seat, is out of the question.
* kids come with a lot of gear (well, they don't come with it. you have to buy it.) while the people may fit, the gear may not.