The problem with moral relativism is that if you let me put my thumb on the scales of good and evil, I can create a hypothetical situation where you have a moral duty to torture a child to death. Say some messed up terrorist threatens to destroy some city or other if you don't. Or it's the only way to keep some ancient demon or whatever imprisoned. Like religious human sacrifice, mandated by the gods and absolutely required to make the sun rise in the morning. Sure, putting bloodthirsty Aztec gods in thought experiments might be seen as kind of unfair, but it's not actually forbidden until you craft an exception for them in your philosophy.
Donating food to starving nations is a different problem altogether. On one hand, it doesn't solve whatever problems caused the starvation in the first place and lets them get even worse so you need to keep donating more and more food and the nation will still be starving. On the other hand, you have food and they are starving, and by donating food you are saving people from starving to death.
Also, saying that good and evil are subjective sounds like avoiding the question to me. Yes, there have been differing opinions on good and evil during history, no one cares about that. Taste in films is also subjective, but you don't tell people that when they ask if you think they should go watch the new Twilight movie. If everyone has a unique view on what is right and what is wrong, why aren't you sharing yours?