Interesting scenario - does he know that lying to the court is a criminal offence?
Revealing you have children doesn't slow down the divorce procedure. In Australia, a divorce is granted if the marriage has broken down irretrievably. You must have been separated for at least 12 months, and the divorce takes effect one month and one day after the application is approved by the court. In regards to children, all he needs to demonstrate is that care arrangements have been made for them. It's a tick box and a space to write 'the children live with the mother and spend time with the father'.
In any case, no, failure to acknowledge children on a divorce application does not forfeit his parental responsibility. Equal parental responsibility is a presumption under the Family Law Act and can only be rebutted under extenuating circumstances (such as extreme and proven violence, neglect, etc.). That means parental responsibility is not removed from a parent unless by order of the court. This can only be accomplished in parenting orders (e.g. Not in a divorce decree).
Hope this helps.