OMG!
That is terrible, I think I would have had to say something then and there, along the lines "Hey, put her down! What do you think you are doing?!!!" but probably would also be so shocked I would have to leave straight away and burst into tears
I second calling 101 for advice. And maybe also reporting the incident to the soft play management [if you haven't already done so].
But it's really really not acceptable and I am amazed that anyone could justify smacking mother's actions.
Yes, she might have been irritated by your DDs behaviour and may have thought you weren't dealing with it [I know that's not the case and you were occupied with DS but just trying to imagine how this crazy woman might be thinking].
But then surely the mature and adult thing to do is either have a gentle quiet word with your DD about sharing or by approaching the parent and asking them to intervene please. Or failing that approach a soft play member of staff for assistance.
But to pick child up and smack her bottom, well that's absolutely not appropriate behaviour from an adult [even child's parents imo] and I think I would be having words with this woman if I ever saw her again.
In the meantime OP have a nice sugary cup of tea and try and relax.
Hope your DD is okay. For what it's worth my DS has also just turned 4 and has suddenly turned into a defiant, aggressive little horror [on occasion, mostly he's as sweet as always
]. Think it might be something about the developmental stage they are at?
Just remember, this too will pass. And you won't be doing soft play hell for ever, thank god!