Flexibility, durability, and easy maintenance are the keys to furnishing a child's room. A NYC interior designer, Stoddard offers suggestions for the main periods of a child's life, with frequent recourse to ball-bearing casters and adjustable shelves and a steady commitment to textural variety, color contrasts, and attractive patterns. Although some of these ideas could stretch the budget--Marimekko fabric, a favorite, is $15.00 a yard--most are affordable in middle-class homes: ingenious clamps for odd items (skis, musical instruments), ethnic rugs and washable wall paints, architect lamps and Salvation Army furniture. And in the interest of congeniality, she votes no to bedroom TVs and telephones, yes to headsets and private territory. Neatness counts--and it all adds up to good judgment, resourcefulness, and a preference for Scandinavian trimmings.