As Occupational Therapists we see children W-sitting frequently.  Children who w-sit tend to have extremely flexible hip joints, and in many situations, have weak tummy and back muscles.  Sitting in this manner provides them with stability when they sit and play on the floor, as it gives them a wide base of support.  Unfortunately, w-sitting impedes strengthening of the core  muscles (tummy and back) and maintains all the structures that provide stability to the hips when walking, running, and jumping, on a stretch.  Here are a few additional reasons why children should be discouraged from w-sitting:

W- sitting in toddlers