Jolie is a unusual bird, but she does spend a lot of time in these poor countries. I really don't think she cares what other people think of her and isn't using the kid as a trophy. She is doing out of real compassion.
Does she still wear that blood vial necklace? Buying kids doesn't make someone very compassionate --- and just to preserve her own figure?
I could be wrong, but that's the impression I get as well.