Quoting Fatty McRunnerPants:" It bothers the smurf out of me, honestly. For two reasons: If my son is throwing a fit it has to do with ... [snip!] ... or try to calm them. Your job is to get me out of the store quickly, helping me parent my children is way above your pay grade."
I suppose it would depend on the cashier, but if i encountered a kid having a fit PLUS a parent having a hard time, I MIGHT try to help if i thought the help had a good chance of working. Sometimes a distraction provides the "break in the action" that truly helps. If i was an employee, it just might make a difference in getting the parent out of the store faster and with less tumult.