They set the rules at their house. You set the rules at your house. The best bet on your part is to let them know what your rules are and they can decide whether or not they want to come over. My concern would be that the dog is not trained if it's taking food from a two year old. Clearly they think that you are overreacting. You don't think that you are and have to protect your dog. That's perfectly legitimate. Lay it out for them. They'll listen or they won't. Just be prepared for them not to come over. My parents don't because I set boundaries and that's perfectly fine with me but you may feel differently.