I feel bad for the child, irresponsible father endangered his life and then having to witness watching his dad collapse totto floor with some angry bloke yelling at him.
Even though he was resisting I'm sure those two coppers could have brought him under control (one looked a lot bigger then the suspect) and put the links on him without having to use a Taser.
But then what happens if the child runs off, maybe runs into the road and gets hit by a car, while the two officers are man handling the guy? or one leaves to go after the kid and his partner gets assaulted by the guy? I've seen smaller guys resort to dirty tactics like their car keys in their fist, etc. before so size isn't everything.
In an ideal world they should really have waited until backup arrived so someone could take charge of the kid but I don't know what the situation was in that respect. Tasing the guy was a perfectly acceptable second best course of action given that he was uncooperative at best.
EDIT: Horrid thing for the kid to experience but there are many ways that night could have ended far worse for the kid.