Harold and Susan Lewin bought a New York co-op apartment and converted the dining room into a living area. Then they divided the original 15-by-26-foot living room into three areas: a bedroom each for the small son and the infant daughter, plus a common playroom.

It was an ingenious plan worked out by Barbara Ross of Dexter Design, Inc., and it is working well for the family.

Son Adam and toddler Gabrielle now have rooms identical in size, plan and built-in furniture so as to avoid any sibling rivalry. Dividing walls are made of plywood partitions. Each child has a window for light and air. Partitions stop a foot or two short of the ceiling, and the open space above them has horizontal blinds which can be adjusted for privacy or opened for fresh circulation of air.

Each room has a built-in counter area, drawers, and shelves lined with laminated vinyl. Extendable lamps pull out from the walls, and assorted plastic bins in bright colors make tidying-up easier. Because the toys, paintings and paraphernalia are so bright and cheerful, the Lewins chose a neutral background of off-white walls and tough gray-beige nylon wall-to-wall carpet. They decided on carpeted floors because of their warmth, acoustical insulation and their safeness and comfort for children who like to play on the floor.

Built-in furniture and two-decker bunk beds are white, but sturdy plastic chairs and storage cylinders in the rooms are bright red. Beds have red and yellow rails and slide-out storage drawers beneath.

The play area has a padded alcove for sleeping overnight guests of the children, or for staging plays, if that's the mood. Mrs. Lewin commissioned a big "soft sculpture" for the alcove, which artist Joy Wulke designed, sewed and stuffed to form billowy clouds, flowers and moon.

The play area also is equipped with built-in storage units, closets for the children and even a haven for dad's bicycle.

The Lewins figure this solution will last until son Adam is about 14. Meanwhile, each child has a room of one's own.