I would go to both Wanaka and Queenstown, and go via the Crown Range to get between them (if you are going in summer). Nicer in a car than a campervan, but still quite doable, and it is very scenic.

Are you pre-booking accomodation, or staying in DOC campgrounds etc? It is nice to have power every second night, but holiday parks are surprisingly expensive once you factor in the kids. Lake Alexandrina / McGregor is a possibility in Tekapo, and the White Horse Hill campsite at Mt Cook is a great spot to hang out if the weather is nice.

If you are playing it by ear then I would choose the route based on the weather. If the weather is good, I would go via the west coast. Maybe even go over Lewis Pass and down through Punakaiki, then Fox Glacier (Lake Matheson is a nice spot in the evening) and over Haast Pass to Wanaka.

Generally the weather is good on one side of the alps at a time, it is often raining on the west coast, and warm on the east. If the weather is not good then I wouldn't go via the west coast.

On the east coast you could go all the way down to Moeraki, then go inland at Palmerston through to Naseby. Or you could go down to Timaru and then inland. The clay cliffs at Omarama are interesting but a bit of a detour (15 mins each way).