Re: Grrr, what a start to the day.
The problem here is that violence begets violence.
Force and violence does not result in changing men's hearts and minds. You win the day, but the guy doesn't learn anything about treating his kid or partner better. In fact he will likely feel so pee'd off and humiliated that he may well take it out on the child/partner with increased vigour.
Self-defence is useful in today's world to protect yourself, but to use it to intervene in domestic violence is somewhat precarious. If it were a Saturday night fight and you tackled 2-3 youths beating up a guy, then you would likely save that guy a lot of harm and the youths would go away and think little of it and they would of course be back the following Saturday for more revelry and fighting. They would likely never see the guy they were beating up again.
Domestic violence is very different. You are never going to get the aggressive male away from the wife and child. Anything you do is going to exacerbate the situation and make life worse for the victims when you are not there.
In domestic violence cases the victim's hearts and minds HAVE to change before the situation can get better. They have to reach a point where their emotions and engrained "love" give way to the rational acceptance that the man is not fit to be partnered, doesn't in any sense love them and once that epiphany is reached they have to take steps to leave/get out.
If you see domestic violence, particularly to children who cannot help themselves, then if possible, film it and take the evidence to the police and report them. Forcibly tackling the guy on this single occasion sadly solves nothing.