A kick to the crotch has the potential of landing on the femoral arteries, of crushing the testes, and of cracking the pelvis. Any of these has potential to, i the right circumstances, result in internal bleeding; a broken pelvis also has significant potential to result in both internal bleeding and fecal contamination... itself a major threat.
A strike to the testicles often results in immediate debility, in addition to the potentially life threatening bleed-out from the rupture, and the high likelyhood of pelvic fracture. Given the ages, the testes should have descended, and the kick would be painful to the point of inducing shock at a minimum.
The strike to the nose, while not inherently lethal most of the time, can, in rare cases, produce sufficient trauma to result in bleeding out quickly. Further, such a strike will often cause a rather immediate whiplash, and possibly a severe, even fatal, acceleration trauma to the brain.
The deceleration trauma when the head hits is another source for concussion and brain injury resulting in death. It's not uncommon for a strike-overbalance fall to result in hyperflexation of the neck on impact... damaging or severing the spinal cord and/or the brain stem.
In short, Card makes certain Ender has multiple plausible, if unlikely, potentially lethal blows, and in combination, they are plenty sufficient for a fatality.
My own read of it is that Bonzo was knocked out by the initial blow, due to a combination of frontal-lobe trauma as the forehead was accelerated into it, pain from the shattered nose, rapid blood loss from the adrenally increased blood pressure, suffered deceleration trauma on head impact, quite possibly to the brainstem, and more attendant shock, and the kick was sufficient to trigger a fatal shock state, especially as no aid was rendered on scene. As he bleeds out internally and through his nose, he's at least mercifully unconscious from the initial head-trauma shock.
I'll also note that the initial head hit is a quite unlikely level of force for an 8 to 9 year old to generate. Still, it's not impossible, just improbable, and ender was actually rather fit by that point.