Blood groups are determined by three alleles

In humans, ABO blood groups are determined by three alleles: alleles IA and IB are codominant and both are dominant to the third allele i. MN blood groups are determined by two codominant alleles M and N. A mistake happened in a big hospital, and parents with phenotypes AB M and B N cannot identify their new born. Below are given phenotypes of five new born kids, one of which belongs to this couple.