In the dictionary, bring means "to take something or someone with you to the place where you are now, or to the place you are talking about" and bring back "to take something or someone with you when you come back from somewhere".
Is the place the difference? "Bring" is limited to the place you are now or talking about and "bring back" any place?
In general I don't see any difference between these sentences:
Could you bring something back to the kids?
Could you bring something to the kids?