jetstar double books flights in darwin 28/3/13

My wife our 4 month old daughter, two year old son and my mother in law where told this morning they could not board there flight as it had been double booked. This flight was direct to melbourne departing at around 0800 28/3/13. This flight was getting into melbourne at 13.30, I dont know the flight numbers etc... I was really looking forward to seeing my family again, and my two year old boy could not understand why he could not get on the airplane, they were there to get on a prebooked flight so why shouldn't he expect to be able to come home. So Jetstar, Is this a regular occurence? do we have to accept this because you are a 'budget' airline? what a joke, actually its worse than that, its highly unprofessional, unorganised and totally diabolical.
Thank you jetstar for stuffing up our easter.

Australia-Melbourne - Tullamarine