This would have driven me apeshit and I'm quite tolerant.
Guy was right. The aircraft should never have been allowed to take off with the child in that state. It wouldn't have with an adult.
The reason for the child being like that is irrelevant to the other 200 whatever people on the plane who had paid for and expected a safe flight.
The mother could easily have had downloaded apps to the iPad to keep him going till the wifi kicked in if that would have helped.
Whole episode was disgraceful and while not the poor child's fault most definitely not the fault of the other passengers. My mother has health issues which would have resulted in this making her very unwell for example. Not just irritated. Staff and parent dealt appallingly with the situation.
I'm still able to have sympathy for the child and realise it was not handled well at the same time.