Cervical spine injuries should be immobilized in a postion of relative extension in both children and adults. The reference by Herzenberg et al found that because of relatively large head size in children, an adult backboard can force a childs' cervical spine into a position of flexion. This might have catastrophic results and lead to spinal cord injury. Therefore, they recommend that pediatric backboards, or modified adult backboards, should be used in pediatric cervical injuries.
Herzenberg JE, Hensinger RN, Dedrick DK, Phillips WA. Emergency transport and positioning of young children who have an injury of the cervical spine. The standard backboard may be hazardous. J Bone Joint Surg Am. 1989 Jan;71(1):15-22.
PMID:2912996 (Link to Abstract)
Wilber JH and Thompson GH. The multiply injured child, in Skeletal trauma in children, ed 3, pp. 73-103. 2003.