Scratches and nicks can be sanded out and the piece refinished to look as good as new. Inspect the finish. No one is tougher on furniture than a child. Make sure it is smooth and uniform, and free of drips or bubbles. Also, inspect interior surfaces to make sure they're smooth.
Quality is in the details.
Signs of quality construction include drawers with dovetail joints, hardware that is straight and firmly secured, and doors that swing freely and smoothly. Inspect for safety. Look for drawer stops that prevent drawers from being completely removed. And, especially when buying for younger children, avoid furniture with sharp, pointy corners.