Dimensions in inches from northamericanmotoring.com....
With seats up in normal position (slightly leaned back) 17.5" front to back and 32" side to side.
With seats down 43" to 47" depending on front seat location front to back and 32" side to side
With seats in secondary up position (straight up) 22.5" front to back and 32" side to side.
Have never tried getting all the stuff that goes along with a toddler in my Clubman, but I know my mate has without any problem. His car will have had 3 adults, 1 toddler, and all her stuff in it.
Nearest I've got is 2 adults, 1 nine year old, and a kids mountian bike. Or two adults, 1 nine year old, lugguage, and enough Christmas presents for 4 adults, & 3 sub ten year old kids.
So with the flat floor removed you'll be surprised what you can get in.
Pop to your dealer with all the stuff you expect to carry around with you. I'm sure any half decent salesperson will let you try and get it all into one of their demo cars
The Clubman D is a wonderful little family motor.