it depends what kind of effect you are trying to achieve. if you want to give probiotics just for maintenance, kefir or yogurt are good choices and probably all you need. i'd give about 1/4 to 1/3 cup per 25-30 pounds of body weight.
if you are giving probiotics as a treatment for certain health issues, kefir or yogurt will not be able to supply the required dose for a therapeutic effect.