That's the conclusion of the 2017 KidsRights Index, which ranks us 158th out of 165 countries. The index is based on data compiled from UNICEF and under the United Nations Convention on the Rights of the Child (to which New Zealand is a party). Unlike other indexes, its scaling system doesn't allow good performance in some areas to compensate for poor performance in others. And looking at the data, the problem is that we do absolutely abysmally on legal recognition of children's rights, getting the minimum score in all six indicators for which data was available:
New Zealand’s record in the 2016 Concluding Observations shows six times the lowest score (on non-discrimination, best interests of the child, respect for the views of the child, enabling legislation, budget, and data). No maximum or average scores are on record. On three indicators the scores of 2016 stayed the same as they were in 2011 (respect for the views of the child, enabling legislation and state-civil society cooperation). On two indicators (best interests of the child and data) New Zealand’s scores improved from non-available to the lowest score. The scores on non-discrimination and budget dropped from an average to the lowest score. With no score available on state-civil society cooperation, which was also not present in 2011, the scores of New Zealand remain incomplete.
This isn't a good performance, and at least part of it is due to National's austerity. Hopefully the next government will be improving on it.