A couple of weekends ago we had my inlaws to stay and also DH's sister and her little boy, who is just over 2 and an active little chap. DFIL has little patience with him!
There was an incident when our nephew was running towards a lamp as if going to touch it. His mum shouted "no" loudly and he stopped in his tracks and turned round. A good 20 secs or so after he'd stopped, DFIL got out of his seat and went and smacked him (not hard) on the bottom - of course he cried.
I was appalled. I personally would rather not smack any child and will try hard not to use this as discipline with our own children. But regardless of how you feel about smacking in general, the timing was awful - he effectively punished him for obeying his mother!
DPIL are keen to help out with childcare when I go back to work after having DC2, but this has really made me think twice about it. DH thinks I'm overreacting and it wasn't a big deal, but I would be furious if he smacked one of our children.