During the final boss fight against King Baby Bowser, it is possible to get hidden stars that count toward the baby timer. In the second phase of the fight, hold up on the D-Pad before firing one of the big eggs. Instead of firing them in the background at Baby Bowser, you will shoot the egg upwards at enemies off screen and a star will drop from the sky. You can do this continuously to build up your baby timer, allowing you to more easily finish the level with thirty stars if you get hit during the final boss.