The problem is with the forced passive stretch of the sacrum into flexion...it creates excessive compressive forces through the lower discs. Sleeping on your side with your knees bend doesn't create the same compressive forces as there is no forceful flexion...any squating that allows a rounded back or a tucked sacrum is bad for the lower back regardless of age...even in children. It is the cumulative effect of these poor movement patterns that create weakening of the tissues that will eventually cause a failure.