Check out these websites! There are many great ideas. There are some great places to visit and ideas that are family friendly, and low cost. Check out some of the weblinks and event calendars to see what you and your family might like to try locally and in other areas of Melbourne.
It's not far from anywhere in the city of greater Dandenong to visit the Cranbourne Botanical Gardens.
Take the train into the city and participate in activities in various locations for example the Arts Centre, the National Gallery of Victoria, Federation square, Scienceworks, the Immigration museum or the Melbourne museum.
The Victorian Government website has a fabulous listing of heaps of school holidays activities around Melbourne. http://www.vic.gov.au/about-victoria-tourism/holidays-daylight-saving/school-holidays/school-holiday-activities.html
For a comprehensive list of family friendly events in Melbourne visit : http://www.webchild.com.au/calendar/melbourne