A women with type AB blood marries a man with type B blood. According to the punnet square, their children could have each of these blood types except

They wouldn't have type A blood or type O blood because if you were to make the punnet square you will see that those two are the only types the child can't have.

it would lead to upper respiratory infections and pneumonia.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).