Just as a thought...what if the gaps were filled in as follows:
3 boys have just been a complete nightmare round the supermarket. Misbehaving, playing up etc.
then they've been hanging off her shirt tales with "I wants" and "Daddy always lets us" etc etc
Then her phone rings and she receives some stresing information relative ill in hospitalbank error, no money--whatever
Then she just snaps...yells at kids because she is stressed up to the limit for a split second....and you happen to see.
What if, 'beat' in their famly just means a flat hand smack on the bottom -- or, (more likely with trashy people), 'beat' is just a threat that is thrown about and never carried out anyway.
You may also have got the 'look' because she knew she was wrong and in that split second, did not need you to point that out to her.
Just saying, that there may well be a whole big picture that you didn't actually see or know about and it may not actually be as bad as you're left thinking it is.
She may well have got them in the car and explained to her boys more rationally and maybe even offered an apology?!
(I know maybe not as well...but maybe)