Mon enfant boit-il l'eau?
I don't understand why there is "il". Also, should it be "de l'eau" instead of "l'eau"?
The sentence would mean My child, he drinks (is drinking) the water. De l'eau would be My child, he drinks some water. Either could be right, they both mean different things.
In French, it is very common to state a noun and then repeat it as a pronoun, as in the sentence you mentioned. It sounds strange in English but quite natural in French. ("My child, does he drink water?" would not generally be used in English -- we would omit the second instance of the subject and just translate the question as "Does my child drink water?")
Juliegolick has it right, that's something as a french-learner I haven't picked up yet, but it's a natural way for people to provide emphasis. "L'eau" would be perfectly correct but "de l'eau" is more correct because adding 'de' changes the sentence to roughly mean "some water" He drinks water vs he drinks some water. This doesn't sound natural in french, but it might help to translate it as He drinks THE water as opposed to he drinks WATER.
I might have just confused myself. Sometimes things aren't easy to translate.