It may be an old thread to some, but I've only just read it, and would like the opportunity to add my twopence, please.
The OP, as far as I'm concerned was definitely Not BU.
In my experience there is certainly a type of parent who treats these soft-play areas as some kind of baby-sitting service for their 'precious' little 'angels'. I'm sure you all know the kind - spends the entire session mulling over a cup of coffee with their mates, or even a book or newspaper, and doesn't once look up to check where their child is, and, nine times out of ten, it's the little brat knocking seven colours of shite out of some smaller child. Frankly, I have to wonder if it's some kind of cry for attention from the perpetrator.
I don't attest to my kids being perfect, and have to confess to having to intervene on countless occcasions when one of my children snatches from, or bashes another. Very occasionally I've missed the incident, but if a child who has been playing with mine suddenly starts crying or is whisked away by their carer, then I'll approach the 'victim' to ascertain what happened and insist my child apologises if necessary.
If the situation is reversed, I'll give it a few moments to allow the carer to register that a 'crime' has been comitted, and if there is no reaction, I will intervene with a gentle reprimand, spoken calmly and as kindly as I can. Young children who don't know any better will then just be avoided.
Persistent older offenders get a different treatment, usually a blood-curdling threat whispered in the ear (something like "if you do that again, i will make sure that you're never allowed in here again") and then if they go running to mummy I'll deal with them as appropriate. Never had to speak to the parents, though, so far, kids seem to get the message!
Overall, however, it's a sad indictment on society that we now seem to be divided into either people who are overly cautious or those who don't give a hoot about other's boundaries.
As, always, it's down to the parents.