The dilemma would be similar to that of precedent. The sum of the consequences would then be compared to the sum of the welfare considerations to decides whether or not to torture the terrorist and whether or not to torture the child of the terrorist. The overarching problem with utilitarianism as a method for decision making is that not enough of the necessary information is available and there is no scale on which to weigh the various considerations. Basically, the subjective utilitarian would probably consider that the deaths of many is worse than the torture of one. Depending on how much weight is given to the detrimental effects of the precedent which would be set by torturing the terrorist, the utilitarian could consider this to outweigh the greater pain caused by the explosion or not.
Different people have different moral consciences, which dictate different actions.
These differences will dictate where the person puts the most weight in the utilitarian considerations, since utilitarianism does not specify. In the end, utilitarianism does not help in making the moral decision. The information necessary to calculate all of the considerations identified by utilitarianism is not available. Furthermore, what is required is a method of comparing and weighing the considerations, and this method is not defined by utilitarianism.