When I went to Winter NAMM back in 2010 or 2011 (can't remember any more), very few booths had "booth babes". There were plenty of women there who knew what they were selling, and some of them dressed up for the occasion while others did not. (Some of the guys wore suits while others wore cutoffs and sandals, too.) Nobody had a problem with this.
That would be my simple defining line. If they know enough to answer questions helpfully, they're legitimate employees no matter how they're dressed or which gender they are. If they don't, then they're hangers-on in some capacity. This is not necessarily a problem, but hangers-on should not be used as eyecandy. If the vendor's kids are milling around, and dressed up in an eye-catching way, this is not a problem. It makes some level of sense that they'd be there, even if they don't really know the business aspect. What should be deemed a problem is hiring random people specifically for the event solely because they're good looking. If they're both good looking and adequately trainable as to be decent sales reps, more power to them.