http://www.yummymummyclub.ca/family/toddlers/20121001/the-dreaded-w-sitting-in-toddlers

This article describes the negative impact of toddlers sitting in a "W" position on the floor. The flexability of little's ones are truly impressive, however while it may not "hurt" to sit a specific way for the child, poor/altered posture results in biomechanical issues down the road!