I'd just like to note that they are not called "девушка" because they work in the customer service. The helper in a store can call me "девушка" too, and that would be fine.
The actual reason is that Russian lacks proper forms of address. There were the words "сударь" and "сударыня" previously, but they were replaced with "товарищ" during the Soviet times. Then "товарищ" became mostly obsolete as well, but "сударь/сударыня" were too long out of use by that point unfortunately, so one can't use them without sounding weird.
Yes, in the US, addressing someone as "boy" or "girl" can be extremely offensive in some contexts, and I'd avoid it. This would be especially bad if you were addressing an adult person of color. But even with a child, I'd be more inclined to use "young man" or "young woman" as a form of address than little boy/little girl.
Depends on the desired tone. Omitting the 'hey' instantly creates a slightly ruder implication. As if an adult was talking to a younger individual whom he didn't think was worth much anything. "Hey boy" can then be anything from very pleasant to very angry. But this is a very detailed difference, as neither is really incorrect or weird.
I agree that "boy, where is your older brother" is not natural. I would probably just say hey, where is your older brother? Or "excuse me, where is your older brother" I would first make sure I have their attention, so that they know I am speaking to them. If I were trying to get their attention quickly, I would probably say "hey" loudly, I don't think I would ever address a child using "boy" or "girl".
*I also just want to say the tone of your voice obviously can affect your intent, like if you were just curious, or angry.
Absolutely, Young man, where is your older brother? Respectful. I answered, young boy, just because I figured Duo wanted Boy and Boy alone is disrespectful and as pointed out, a slur. You might say, Little man or Little boy. Boy alone is ONLY disrespectful. Tarzan called his Son, BOY to show he was primitive with crude language skills!