How young is the pony, and what and where was he hit with? What do you mean by being resistant? Did it cause the pony and fear, distress or discomfort? How did you know it was done out of temper?
In my view it is not abuse to reprimand a pony with a light tap, with an open hand, especially if they are behaving in a dangerous way (and if he was waving a front leg around, for example, that can be dangerous). Horses do kick each other much harder than we could ever hit them. Legally, it would only be abusive if she caused the pony fear/distress or lasting discomfort and pain (by lasting I mean lasting past the initial sting of a slap). If you believe this to be the case then that is different and you should have a word with the yard owner.
Hitting out of temper is wrong, but working in a riding school can be really frustrating at times, and it's not always easy for senior staff to get a few minutes to cool down if they are feeling stressed. Perhaps in the future it might be better to offer to step in and help, rather than watching and judging. If you're not experienced enough to do this, then you're probably not well placed to judge if the remprimand was necessary.
I do think having a lot of children watching someone deal with a young horse is not ideal for lots of reasons though.
If you have real concerns, then by all means speak to the yard owner, but please do bear in mind most people do use positive punishment when dealing with horses, due to their size and potential to be dangerous. This is not the worst you will ever see in the horse world.