Whakapapa is offering free season passes for those under 10 year's old during the pre-season period (ends 17 May). Other age ranges are also discounted.
For infants (0-4 years), they remain free in season.
For Child (5-10 years), in season, the season pass price goes up to $499 ($399 for weekday only), and day passes are fairly substantial.
I missed this until today so thought I'd post in case someone was in the same boat!