Kind of difficult for me to advise a course of action to take having not actually observed the situation personally. I've been in some circumstances where workers would "insult" each other and it was more of an initiation, a way of bonding or tying to bring new people into the fold. Those who didn't take the insults personally and could dish out/join in were accepted, those who took it personally misinterpreted the intent (and the others couldn't understand why they were taking offense). In those cases, I would have to explain things to both sides - telling the one being offended that wasn't the intent, and telling those joking that in reality, what they were doing isn't always interpreted as "joking".
I, myself, am short, so if I was called a "midget" - first, I'd be sure the little person truly wasn't offended. If that was just their sense of humor, then I would respond to taller people as "Jolly Green" or "Amazon" and ask how the weather was "up there" - just poking fun as "how tall" they were in comparison to "how short" they were implying I was. Although, if the jokes continued to the point of boredom or were interfering with my work, I'd plainly state I wasn't interested anymore.
For the "kid" who thinks it's funny repeating you like a 3 year old, I would flatly ask if he needed to be put in time out. If he wondered why I asked that, I'd state a 3 year old acted more adult than he was, so if he was going to act like a child, you may as well treat him like one.
In general, I've found group dynamic situations like this often mimic basic animal dominance behavior. Take some lessons from the show "The Dog Whisperer" and employ some "pack leader" strategies to take control of the situation. Generally, the pack leader uses calm assertiveness to take control and lead the pack. A pack leader doesn't get emotional, remains calm and asserts control by ignoring the antics of the pack until they conform to them - using tactics to make them conform.
Perhaps things like with the "kid", just order him to do a task. Even if he mocks your request, just look at him with an "I'm not joking" look and say "just do it." then go back to your things. If he doesn't, assert later he was told to do a task and failed - "now go do it."
Eventually you'll be deemed as "not fun" because you are not getting riled (if that's their intent), and eventually people may actually start working.
Sometimes that's easier said than done. I always tended to lean toward the calculating and logic thought, not a highly emotional person myself. If this is something that would be difficult for you, then the next course of action would be to have a sit down with your manager. My advice for this meeting would be to report your issues when you are not emotional - just state the facts, how the behavior is negatively impacting your productivity - and if the behavior is generally negatively impacting the store's bottom line. That will mean more to management.
Hope you find a way to get thing smoothed over one way or the other.