Please tell me what are reasons why a mother or father cannot donate a kidney to their child?

Tissue typing: The degree to which the body will mount a challenge to a transplanted organ depends on how closely the tissue immune markers match up. Just because you are a parent, does not mean the blend of genes that occurred with conception will produce a kid that is compatible with either parent. It can happen, but is not always the case.The best match possible gives the best chance of success.