On a recent flight from MEL to HKG, which also happened to be a 9-hour day flight, I saw a family who was separated between J and PEY (or Y). I am not sure if half of them were seated in PEY or Y as I was in the front row seats of PEY and never bothered to turn my head and see where they were seated. I am almost certain that one of the family members is a diamond member as I saw them checking-in and their luggage was tagged for first class priority. However, the point I wanted to make was, for the entire flight, the kids were walking back and forth between the cabins, with the mom carrying snacks over from J to their seats from time to time. At one point, one of the FAs even carried one of the kids between the cabins so that they can see their parents during the flight.
As a passenger, I felt sorry for the FA and I only find this to be disturbing. If they want to be seated together as a family then they should buy/redeem their tickets in the same booking class. It doesn't matter if one of them is a diamond member. It's nice that the FA was being attentive and went out of her way to take care of the kids and the passengers, but this should not be taken for granted and cannot be expected as an automatic gesture. It was disruptive to other passengers when people keep walking back and forth between cabins as that inevitably will create noise and inadvertently bumping into other's seat/hand rest. I felt sorry for the FA who had to entertain this "diamond" family to such far extent, as if they don't have other cabin duties to attend to.
Going back to the original OP, I am not implying in any way that what I witnessed was the same group of family. However, if you want your kids to be seated together then you should buy/redeem the same cabin class. I agree it is rather disappointing if your IFE didn't work for a such a long flight and certainly that's something CX needs to work on, but I would imagine if this happened to the entire cabin and if you filed a complaint, CX should provide some sort of compensation. Unfortunately, I do not think the ISM has any authority or power to give you free Wi-Fi access though.