If your kids like animals, Healesville Sanctuary is a great place to visit:
http://www.zoo.org.au/healesville
Seeing the penguin parade at Phillip Island is also great:
http://www.penguins.org.au/attractions/penguin-parade/
Great Ocean Road day trips are LONG days....usually setting off from Melbourne CBD around 7-7.30am and not getting back until 9pm. Doing it over 2-3 days would be better if you can afford the accommodation costs and self-drive.