i read that babys shouldnt fly till they are 6 weeks old, something not quite developed enough to handle the pressure change - maybe ear, or inner ear.
And like Sarah said, give bubs dummy, feed etc on take off and landing. My sister flew Melb to Perth when her boy was about 7 or 8 weeks, and he was fine. No probs at all.

ETA - how do you drive to TAS?