First, I dont in any way think that shouting at a two year old to fuck off is okay.
But and its a big but a lot not all of the people here are saying, "well I never shout at my nine month old". There is a lot of difference between shouting at a baby who cant understand and shouting at a naughty toddler who could well be doing something dangerous. I have shouted at my toddler, same as an example given above, when he opened the front door and ran away. I have never shouted at him for waking up (though I may have shouted at DH for moaning. Hes a baby. They wake up. Deal with it DH!)
From what you've said, yes it was the middle of the night, but you dont know they were shouting just for waking up?
Another thing... Smacking is not the most ideal discipline technique. But its not illegal yet. And a lot of people still do it. So threatening a smack alone, is not abuse. Could be that "light tap" that MNers are always on about.
But at the end of the day, you were there, you know what you heard and you are the only one who can put it in context with their normal behaviour. So if you think you need to ring the police/NSPCC, then don't faff about worrying about it, just do it.
Not ringing isnt a bad thing. Ringing isnt a bad thing. But thinking you should, and not, could turn out very badly.