Why is it not "Mon enfant boit l'eau?", without the "-il"?
because it's a question! think of "-il" like adding "does"
otherwise you'd be saying "my child drinks water?"
What about "Boit-il mon enfant l'eau?" Seems like that should be just fine.
Is "Boit mon enfant du l'eau?" not OK?
No, at least say: de l'eau. de le > du, de l' > de l'. "Est-ce que mon enfant boit (de) l'eau?"
Boit mon enfant de l'eau?
Is that correct?
I would never say this.
Wow, this site is filled with errors! I advise you to not continue your French education through this site.
You HAVE to say "de l'eau". Otherwise, you're implying that your son drinks all of the water in the world.