What happens when you immigrate to Australia and hide the fact that you have an illegitimate child?

This has no effect on the immigration application, because you have not fabricated facts and provided false materials. However, your illegitimate child will be excluded from the immigration application. If you want to immigrate your child in the future, it will be very troublesome. Immigration will question why you hid it in the first place Will wonder if this child is your child after all?

It doesn't matter if you have no children.