In order to answer the question in moral terms, I start asking questions. "How many innocent people will be killed?" "To what degree are the people killed actually innocent?" "How many innocent people will be killed if the action is NOT taken?"
The terrorists targeted by the drones are in the very business of killing innocent people as that is almost their raison d'etre. In addition, the people who surround them often share their same objective. Children are always innocent, of course, but I don't think it is entirely cut and dried as to the innocence of adults.
THe morality of drone strikes is very complex IMO, and doesn't lend itself to simple answers that ignore too many variables and fails to ask enough questions.