Planning to go for south island self-drive family vacation trip from 12th - 23 march 2019. Three healthy adults, two elderly with knee issue and one children. Do you guys think the dates are ok?

The route that i got in mind now is:
Christchurch - dunedin - invercargill - te anau - queenstown - twizel - christchurch

Initially was thinking to cover haast - hokitika - arthurs pass and the glaciers. However, this require hiking which i dont think my parents is able to do it.

Appreciate your advise and sharing of experience.