My two youngest, ages 6 and 9, share a room. Two under-bed bins at $25 each solved most of the clutter. It works beautifully now. Getting there required solving several specific problems, the biggest of which was bedtime.
The Bedtime Light War
My 9-year-old reads before bed. My 6-year-old needs complete darkness to fall asleep. For six months this was a nightly battle. The solution was so simple it's embarrassing that it took us this long: individual wall sconces on each side of the room, on separate switches. The 9-year-old reads by her sconce. The 6-year-old's side stays dark. No overhead light. No conflict. Everyone sleeps.
The Zone Rule
Each child has a zone: one side of the room, their own dresser, their own shelf space, their own side of the shared closet. The zones are mirror images of each other, same furniture, same storage. Nothing creates more friction between siblings than perceived inequality in their physical space.
The Neutral Zone
A shared bookshelf in the center of the room holds books and games that belong to both. The center of the room is communal, no one's territory. This boundary is actually respected better than I expected because the personal zones are clearly defined.
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