Fallingskyward:
Why do you hold that murder is a greater sin than disloyalty?
In this situation, I hold that murder is the greater evil because you would be causing extreme pain to an innocent child in order to follow through with an unethical pact you made. I do not believe that your personal blunder in making the pact is enough to warrant following through with a heinous task.
I concede that the ethics in the situation become more ambigious when you add more to the circumstance, such as if you had to kill a random child in order to save a larger number of innocent people and had no opportunity to avert the situation(such as contacting authorities). The best choice would probably be whatever causes the least amount of pain, but that kind of circumstance is incredibly unrealistic. The initial hypothetical is somewhat plausible - Charlie Manson's girls come to mind. But these people were unethical in the first place, as is anyone who would follow through with a pact like that without some extremely extenuating circumstances.