Kids with SPD are usually (not always) prone to w-sitting due to weak core strength and/or low muscle tone. You can "correct" the sitting as much as you like (straighten your child's legs out in front of him/her, or manipulate them into the "crossed-legs" position), but you are treating a symptom and not the cause. You will have to help improve your child's core muscles with focused exercise. You can either (1) do your own research to develop an exercise programme at home, (2) ask an occupational therapist or a biokineticist to develop a programme for you to follow at home, or (3) alert your child's Occupational Therapist (if your child is seeing one) to the fact that he/she is w-sitting and ask that this be addressed urgently during sessions (a home programme will be provided too). Hein-Chris used to be the poster child for w-sitting as a toddler, but after many visits with his OT, and lots of exercise at home, he very seldom will revert to w-sitting these days, and, when he does, he'll correct it himself quickly.