A murphy bed will get really tedious, really fast. It also means their bedtime is your bedtime.

A boy and girl sharing is tricky, and your time-scale of 5 years is maybe pushing it. (Not a parent, so I can only think back to childhood, and I would have hated sharing with my sister.)

Is there something special about this particular apartment? If not, the simplest (ha!) thing to do is to plan to move to a purpose built 3-bed apartment in a few years if at all possible. Unfortunately there are far fewer 3-bed apartments usually.

Good luck whatever you decide.