Think you'll struggle with the boot and a double buggy. Had a 1 series 5 door when my son was bor, and the rear facing child seat (maxi cosi cabrio) was right up against the back of the front passenger seat, even with it moved forward. But we managed. However, we have a bugaboo buggy, which is pretty big, and, when configured as a pram, it completely filled the boot. Once we changed to to buggy config, it was better, but you may struggle with the convertible, especially with the roof down.
However, don't let that put you off! They are lovely cars IMHO. Where there's a will there's a way
Babybjorn carrier for the youngest and a lightweight maclaren buggy for the other one maybe?