The interior walls can be
covered with the plywood sheet at any time once the roof is finished,
building wrap (cover underlay) envelops the structure and the door and
windows are installed.
Cover the interior walls with 7mm (11/32") or thicker plywood. Nail to the studs.
Cut out for the door and windows where necessary and also
cut the plywood sheets
so that any join will be over a stud.
A standard 1200mm (48") wide plywood sheet will reach from the middle of the bottom plate to the middle of the top plate.
Finish with trim around the windows and door, around the top of the
walls and around the bottom of the walls (baseboard, skirting).
Use 20mm x 20mm (3/4"x 3/4") wood around the windows and 50x25 (1x2) wood for the rest.
A bit of paint, a few curtains, a piece of furniture and the kids have their own private retreat.