In reply to mike1979:
Summerday Valley is probably a good place to start...2 minute walk in nice comfortable and flat cliff base but you will need a decent trad rack. Further up to van Diemens you will get some more sport routes but this is a steep (in parts) 30-40 minute walk in and some tricky ground in general (lots of drop offs). Stapylton generally eg Hollow Mountain is a great place for exploring but it could be tough to combine sport + easy access + kid friendly. The Gramps in general have long walk ins.
I only know a couple of the main areas (there are about a hundred different crags) so maybe try looking on the crag:
https://www.thecrag.com/climbing/australia/grampians
or ask on chockstone. At the more obscure sport areas, definitely be alert to holds breaking off.