I was busy doing something that required both hands.
Baby started crying in her pram, toddler went over and started stroking her head but then this changed to hitting her (toddler has form for this when tired plus overwhelm of baby crying. I genuinely don't think it's malicious, more strange toddler impulse, we are working on it, it's improved, and we always physically separate them if this happens, it's not ok to hit etc etc)
Before I could get there my mum tried to diffuse the situation by telling her to stop and then I heard her say 'do you need to see what that feels like?'
At this point I just dropped what I was doing came over, took my toddler (baby strapped in buggy to easier to remover toddler) and told my mum not to threaten her.
Mum said 'i wasn't threatening her'
I said 'the only way to show her what that feels like is if you were prepared to hit her, which is a threat'
She said 'its not a threat I was teaching her lesson'
I think, even if my mum knows she would never actually hit her, as far as the two year old is concerned her grandmother was prepared to hit her.
For context we have a difficult relationship, my mum can't see things from anyone elses point of view, will never apologise or acknowledge what she could have done differently, will defend her actions until she's blue in the face - so she won't see this from the toddlers point of view only hers which is 'i was never going to actually hit her so it's ok' - and she won't see this from my point of view but will accuse me of overreacting. So I'm not bringing it back up with her.
YANBU: it was a threat and it's not ok to threaten a small child with physical punishment
YABU: It wasn't a threat/you're over reacting
PS. not looking for advice on the hitting please, just did my mum threaten toddler or didn't she?