Here is the regulation. https://www.immigration.govt.nz/opsmanual/#46161.htm You will see that the child will not need to have an X-ray.
Here https://www.immigration.govt.nz/docu...ry/inz1216.pdf is the INZ instruction to doctors for the medical examination. Notice particularly Appendix 2 about developmental milestones (and that there is very little expected of a child as young as yours will probably be at the time of the medical).