It might be more convenient for the present applicant, as the parents/grandparents can choose to use the 18 months almost all at once (with brief visits out of the country each six months), which would mean they could be there during their grandchild's very youngest years.
However, BEWARE. Childcare and other household help, even for a family member, even without pay, is seen as WORK by INZ, which is not allowed on any kind of visitor's visa, and mention of this can lead to the visa being cancelled, and problems for any future visa application. So nobody should plead their case for extra visa time by saying, 'Oh, but my Dad is half-way through fixing the deck,' or, 'My Mum fetches the children from kindy while I'm working.'
https://www.enz.org/forum/showthread...801#post515801