How do you disavow a marriage where there were children?
The marriage is not disavowed, it is invalidated. No one denies the children were the result of two Catholics and they are not bastards either. Some marriages fall apart because not everything was known about one spouse by the other up front. The children are not to blame for this and neither is the spouse that was wronged.
An annulment says that there was not a valid Catholic wedding due to circumstances.
Couples were married and children are not illegitimate.
It just wasn’t valid to begin with.