If it was "front license plate to front license plate", this was a full head-on crash, and not a crash that the IIHS tests; they only test offset collisions. The NHTSA tests full head-on collisions (safercar.gov). Both the RX 450H and the Equinox received 4-stars in head-on collisions. The Equinox only has front airbags while the RX 450h has front and driver knee airbags.

In front offset collisions (both moderate and minimum offset collisions tested by the IIHS), structure is very important. In the full head-on collision, however, structure matters less than seat belts and airbags because the structural strength of all vehicles sold in North America is strong already. American vehicles have been known (at least in the past) for allowing some slack in the seat belts. Too much slack could cause a (more severe) secondary collision: the primary collision is when the vehicle hits something and the secondary collision is when the person hits something in the vehicle.

The death of the boy may have been caused by inappropriate use of a booster seat; the boy may have been too small (not heavy enough) to use a booster seat with the vehicle's 3-point seat belt (with slack) when he should have still been in a proper child seat with a 5-point harness (and no slack if properly belted in). If he was heavy enough to properly use a booster seat, some slack in the seat belt could have been the cause of his death. But this is all speculation.

These tests (both the NHTSA full head-on and the IIHS moderate and minimum offset tests) crash the vehicles into stationary barriers that is equivalent to two vehicles of the same weight colliding. I do not know if the Equinox and the RX Hybrid are considered to be in the same weight class; the RXh is a heavier vehicle and in a crash with a lighter vehicle, it would win out.