No offense but your friend has NFI

I know a few kids who have decided they would rather share a bedroom than sleep in their own rooms. And that size room for 2 kidlets won't be a problem gosh thats nearly as big as our master bedroom and could easily have separate spaces.

I think it will only become a problem as the eldest starts entering teenage years but by then you may want to add an extension on or something.

No you're not being selfish at all