There’s plenty of space in the back of the X1. True, there isn't quite as much head or leg room as in a VW Tiguan, but a couple of six-footers will be more than comfortable on long journeys, especially with the sliding rear seats slid as far back as they'll go.
As is the case in the X1's main rivals, carrying three adults in the back of the X1 is a bit of a squeeze, but it is certainly possible. Three kids in the back will be perfectly happy.
Storage space is pretty good, too; the nets on the backs of the front seats make for convenient document holders, and if you pay a bit extra you'll also get folding tray tables on the front seat backs.