Rueda et al. (Neuropsychologia 42/8; 2004) found that children cannot keep two things in mind at the same time until after age 7. (They used an Eriksen attention task.) Have you accounted for the brain links that need to grow before children can think about length and width at the same time in dealing with area? For middle/upper class children, area/volume will begin to make sense at age 8 or later; for children growing up in the toxic stress of poverty (see Early Childhood studies from Harvard), that brain growth is delayed. Children from low income families may not have the ability to keep two things in mind at the same time until age 9 or later.