Luckily we have laws to protect us against people who do, regardless of whether you feel that is overboard. Where do you draw the line? How violent should someone get before the police are called?
In answer to your question "Where do you draw the line?" I don't think the barometer is based on "How angry am I/how violated do I feel?" I think it's more a judgement call of "Can I resolve this situation myself?" Sure there are things that we can ask of law enforcement, but should we?
Police resources are limited. I want the police to be available if my car gets stolen, or if I'm in a car accident, or if someone is stealing something. I think it's ridiculous to threaten or even contemplate involving police for a poorly-considered salesclerk decision such as removing a child's hand from a display.
As an aside:
I'm a little confused by your description of events (not that it okays her response). If your son was RIGHT beside you and likely touching the item in an attempt to show you, how did the salesclerk even get close enough to be touching him and sticking her finger in his face without you noticing? By the time she got that close to him, wouldn't she have been invading your personal space? When I am shopping, I stop inspecting items and look up to respond to the approaching clerks. They wouldn't even get within arm's reach of me.