With 7 children I would definitely pick the cheapest flights!
We flew out here initially on Air New Zealand (from Heathrow to Auckland) who were great, we have only 1 child to entertain though.
What I would advise and this is really obvious I know, is to make absolutely certain that you have seats together or at least in 2 groups. Our flights were booked and paid for by the NZ employer and they left it so late (about a month before flying) that we couldn't have guaranteed seats together and had to basically be first in the queue. All worked out fine in the end but unnecessary hassle.