Something like Melbourne to Sydney or Melbourne to Adelaide is doable but it depends on what the family enjoy doing and how old/young the kids are.
Tasmania is very outdoorsy and distances are large. Assuming you are going during easter or our school hols it’ll be Aussie winter so might be better staying on the mainland.
We had a very short timescale but managed half the GOR, back past Melbourne to Sydney via the coast road, blue mountains and Sydney itself in 10 days in a campervan.
If you like wildlife, I can recommend Kennett’s river, Vic for koalas, Durra’s north, NSW for parakeets and kangaroos and glenbrook campsite, blue mountains for getting close to roos.
There’s plenty to keep you occupied around melbourne – GOR & waterfalls and forest inland, city & beaches, Phillips island penguins, vineyard tours, alps.