Melbourne CBD to Phillip Island and back to Melbourne CBD?

My family of 3, including myself, is interested in visiting Phillip Island -- what are the best train routes and fares for a round-trip from Melbourne CBD to Phillip Island and back to Melbourne CBD? Is there a suburban metro or train that we can take? We are also open to the option of taking a bus to Phillip Island and back as well, if time and price favors this option. Which stops to we get on and off? We're looking at spending the first half of the day in Melbourne, and then going to Phillip Island in the late afternoon/early evening to see the Penguin Parade, then returning back to Melbourne in the late evening. What stop should we get off at on Phillip Island, and what is the entrance fee for seeing the penguins? Please advise, and thank you!