One difference is that people organizing the child-soldiers of, say, Sierra Leone or the drug cartels using minors as couriers are themselves immoral and heartless scum. It's quite a different proposition for a character you're supposed to admire and respect to willingly place a minor in extreme danger. Wartime breaks nearly all the rules of moral behavior, but voluntary vigilantism is a different thing altogether.
There's a huge element of wish fulfilment in super-hero comics from the basic premise, and readers have to decide where they draw the line on how their heroes behave. Using children too young to drive by themselves to fight armed killers is where I think I draw the line as unacceptable.