If you only use a head rest and not shoulder and rib support I feel you do more damage in side impact. Without the should and rib support the body moves sideways and the head stops at the rest causing more damage then good. If child sits at about a 15 degree angle (not leaning outside of cage) but leaning with spine and neck all in alignment the helmet will be easier to support. My 6 year old weighed 40 pounds and does fine without headrest.
Just my opinion..