I’d need to hear more details too. A kick in the belly wouldn’t be my plan personally but when a horse is being a brat, a good assertive smack is warranted. I don’t think the horse looked at you in fear, I think the horse probably looked at you like “oh sht!” I have a young horse too, and there’s a difference between being understanding with them because they are young and letting them be pushy and bratty. So no, I probably wouldn’t have kicked her in the belly. But also she might’ve needed that “oh sht” moment because it sounds like you might’ve been letting her get away with too much.
Now, this said, if this were my horse I would not want to be at a barn that had vastly different ground manner standards than me because that’s not fair to the horse. You can’t let your horse get away with things that the staff and BM will not. If you don’t mind the pushiness and dancing around that’s on you, but it’s confusing to your horse when acceptable behavior is all of a sudden unacceptable. When you talk about “that look of fear” I don’t think that’s what it was. I think that horse was just so used to being allowed to dance around and be a brat that when someone told her NO, she was a little shocked and confused.
So, I see 2 options here. Either you raise your ground manner expectations for your horse, or you go to another facility with similar expectations to yours and where they don’t need to handle your horse on a regular basis. It’s just not fair to the horse or to the barn staff if your horse is acting like a brat all the time and you’re not doing anything about it.