Seriously?The scenario is highly circumstantial. Proximity to me, ability to swim (dogs are inherently better swimmers than humans), which one I have the best chance of saving, etc.
I would save the child because I am wired to have greater empathy for my own species, and can think of nothing in Judaism that might dissuade me. The thought that I might pause, evaluate distance, water current, wave chop, natural swimming prowess, etc., before making a decision fit for a forum thread strikes me as absurd posturing at best.