Milford Sound is in Fiordland, not on the West Coast. The Pancake rocks are unlikely to entertain some littlies, and the glaciers would probably leave them cold too. (
) Seals aren't that easy to find in most spots on the West Coast, and beaches strewn with driftwood may get boring after a few days.
But it is very atmospheric much of the time, although that may be lost on small children, especially after days and days of rain and sand flies.