The ABO blood group system of humans is an autosomal trait that has three alleles as follows - VCE - SSCE Biology - Question 25 - 2013 - Paper 1

Given that the children have four different blood groups (A, B, AB, and O), one possible combination of parental genotypes is AB (which has both $I^A$ and $I^B$ alleles) and O (which has the $ii$ genotype). The AB parent can provide either $I^A$ or $I^B$, while the O parent can only provide the $i$ allele. This combination allows the children to inherit a range of blood types, consistent with the stated conditions.