I have two mixed chihuahuas. They are very friendly but have become a little more aggressive over the last year and a half, beginning when I became pregnant with my second child. They bark and growl at my daughter when she comes near them. We have been teaching her to respect their space, but it happens in more than just their areas of the house.
They were house trained, but have been marking since we moved homes. We moved just before the baby arrived. We have been trying to retrain them, but we have not managed to stop them.