No, it won't backfire, it's actually expected that you talk about what you would want to specialize in. Programs expect a certain percentage of people to sub-specialize and in the more academic programs it ends up being > 50%. Why do you think it hurts to mention that you're interested in sub-specializing?
It's fine and expected. The only exception might be if you happen to be interviewing at a small community program whose mission is strictly to train the next wave of primary care pediatricians in the area, but then I would wonder why you were interviewing at such a program in the first place if you're sure you want to subspecialize.