consider sensory and perceptual difficulties, this may be a factor in delaying independent walking. Children with Williams syndrome often have a real strength and liking for music. Perhaps try and bring this into a session, or better yet, link up with a music therapist and work together. Also the children are often quite social and may respond well to a group situation. They often like to avoid activities too challenging, make it fun, give extra sensory information and perceptual cues. Good luck, I would be interested to know if this helps