At what point do you step in?

We go to a local indoor playground regularly and see a specific mum there at least once a week. This lady is obviously at the end of her teather. I am not judging her, I know where she is at and I was there not long ago but I do wish someone had stepped in and offered help which is why I'm asking. How do you approach this situation? How do you offer to help a mum who obviously has no desire to 'mum' anymore? I'm talking yelling at her son for the smallest things, constantly staying "oh for f*cks sake, I'm so f*cking sick you, you're an f'ing a**hole" etc to her ~3yo son. He took a car off my daughter, no biggy she didn't cry, it's what kids do and the mum went running over and crabbed her child by the arm and yelled and smacked his bum (hard and out of anger, not discipline). Seriously I was nearly in tears watching the situation, I feel for the child but also the mother because I personally know how exhausted she likely feels but I just don't know how to help!

Please note this child is not neglected they are clean, well dressed, eating nutritious food- I don't think calling child services is an approximate answer.