It is an old thread but aside from whether you agree with polygamy or not, there are plenty of western people who live in such relationships too although they can legally only marry one person.
If someone has more than one wife, immigration would only consider the first marriage legal. However, immigration requires that all children be listed on the application, regardless from which wife. They must be listed as either accompanying or not accompanying and they must be medically examined in order to either get a PR visa or to keep the option open to sponsor them later.
Children who were included on a PR application as not accompanying may be sponsored for PR at any time before their 19th birthday.
If the man was already a PR when the children were born, he does not have to notify immigration about the birth of the child but still has the option to sponsor them before their 19th birthday.
If the man was already a citizen when the children were born, he can apply for their citizenship.