I think this is a personal choice. there is no a catagorical answer. well unless you are Kantian in your moral
anways, there are many ways to look at it.
I'm going to try to do it wihtout the subject of God as it complicates the issue greatly in my opinion
so this is a non-god answer to your questoin.. hope that doesn't defeat the purpose completely
To aristotlian (i can't spell for crap -__-), i think it's the matter of consistency thus, your actions are defined by habbits not the motive at the moment. thus your moral righteousness as a person is defined by how well you are educated/trained since youth to perform acts of sobriety as habits. he believes a accidental performance of good acts doesn't mean anything. thus you can also argue, your motives are really important in that aspect.
*** to aristotle, to save the kid is what you are taught to do. if you are ethical, you will do what is right without a second thought regardless of you motive. you are taught to do so and you will as excellance is a habbit...
moving forward in time, in the enlightenment age, as i pointed out earlier, the dominent moral would be Kantian. In the kantian sense, dicipline and absolutism is the game. "Act only on a maxim that you can will to be a universal law." your motives are especially important because through every decision you display your ethics and ability to act thrgou hte catagorical imperitive.
*** to kant, saving a child is regardless of his statue. you save a person from drowning regardless of who they are. it is a "sin" to let a criminal to drown as to let a child drown. motives are important but laws are nevertheless absolute in the Kantian sense. thus you have to save the boy regardlessly
if you move to Marxist and Mill's utilitarian type of moral, they deal more with politics... so i probalby should mention it here. but to Mill's and Marx, it is also important to understand you motives.
*** Marx would say your doubt in helping someone for an reward itself may be unethical because it is a form of alienation from another person.
*** Mill's would say saving hte boy is a great thing even if your motive is money
because it's a win win thing. you are happy, he is happy.
*i love mill :P * as long as your action brings the most amount of happiness to the most people, it's a good action.
my knowledge on ethics and moral is very limited as i'm only graduating from high school this year -_- if you want more ino:::
http://www.iep.utm.edu/
hope that helps