When is allowing W-sitting ever a good idea? Pretty much never. Why? It limits the growth of the child’s neural functioning (limits the transfer/flow of the 2 sides of the child’s body/brain), limits the development of their baseline awareness/use of their core muscles and it has a chance to be harmful for the development of the child’s hip socket and knee joint.
Children with altered muscular tone are more prone to W-sit. It is a symptom of atypical development and muscle weakness; therefore, it should raise a concern to the discerning parent/child care provider.
Most children who assume this posture started when they were infants and this postural “negative habit pattern” if not addressed will persist though out their development. What should one do? Offer alternative sitting positions/postures. You may need to physically help the young toddler to use an alternative position i.e., criss cross legs, or “P” or “q” posture (one leg extending out while the opposite leg’s foot bottom is touching the extended legs thigh). If your toddler/preschooler is a persistent W-sitter have them sit up off the floor i.e., cube chair and don’t be afraid to use the cube chair at “circle time”.