At the moment the Spirit of Tasmania has a promotion on that allows you to take UP TO three children on the Spirit of Tasmania, for each Paying Adult Cabin Fare.
Their website states
Little people now cost very little. Because for a short time, kids travel free. Just book an adult cabin fare and up to three kids travel free!
The Spirit of Tasmania Travels between Melbourne, Victoria and Devonport in Tasmania and offers a wonderful trip allowing you to bring your car if you choose for a small fee.