Recently flew from Auckland to Perth economy class with 2 adults and 2 pre schoolers. We booked for sky couches with 1adult and 1 child per sky couch.
It was well worth the extra cost.
The kids (and us) had enough room to stretch out and relax. It turned what I thought was going to be a nightmare trip into an almost enjoyable one!
I think this would also be quite a good option for longer trips to USA or Europe as well and we will definitely be getting sky couches for any medium to long haul flights with the kids in the future.