Medical for husband's Kids.

[Mb80]

Hi i had applied for a spousal sponsorship for my husband, he received his medical request and immigration has asked for medical exam for his children from his first marriage as well though the children are in their mother's custody. Their mother is not willing to cooperate in doing the medical for the children. my question is, how should we explain this to the immigration as it's clearly mentioned in their divorce deed that the children are in their mother's custody. Will this result in refusal of visa for my husband.