I agree, some massive overreactions here, by lots of people acting like they have never got cross and behaved in not the best light.
I doubt dad was just angry she was taking too long to eat/drink before bed. Does no one think kids of that age are clever enough to purposefully drag out the time it takes to do a task before bed, to avoid bedtime? Give over. I tried it on when I was a kid (and got put right pretty fucking quickly!) OP said herself 'she was ignoring him', and from the tone of the post I'm willing to hedge a bet it happens a lot that she ignores her parents, and mum defends children, and dad lost his temper today. I also think he's being treated a lot differently than if it were a woman that lost her temper with her child, going from posts I've seen, including ones where they've smacked their children, and still been told by mumsnet 'it's okay, no ones perfect'.
There's a lot of bratty-ass children being raised, to ignore their parents/those responsible for them. I don't think the go to answer should be dragging them across the room, of course, though I can certainly see how it happens to children who choose not to respect what their parents say and behave obnoxiously toward them. Parents are, after all, responsible for setting rules for children. The kids don't get to choose on some of them, thems the breaks.
I'd be questioning why she was ignoring her dad, and what I and he could do to prevent that behaviour next time, and move on from it. I have a feeling they take the piss out of him, and you encourage them by being soft on them.
I don't condone hitting, dragging, or anything of the sort. Simply saying, I understand how it can happen, this wasn't a smack. He told her to do something, she ignored him, he took her arm and walked fast the direction he'd told her to go, and her arm got accidently knocked. Maybe none of this would have happened if a) you and him were on the same page as to how to deal with these behaviours, b) he maybe educates himself on how to deal with the behaviours and c) then you together, work on your children not ignoring their parents.
I think you and your husband need a good chat tomorrow. His behaviour needs to be addressed, but also yours and your children. See how, as a family, you can work together to create harmony.