Our energies are magnetic in nature. Emotion is energy in motion. We can attract energy to us -- including the energy of people we do not even know -- when we are not safeguarded from it.
Children process a lot of negative energy for their parents. If you are feeling stressed, your child is intuitively aware of that and can play a role of trying to "please" or "act-out" in response to the negative energy you are running: http://www.mercola.com/2006/jun/29/how_to_protect_yourself_and_your_children_from_the_negative_energy_of_others.htm.