Thanks for your question. Balloons such as the kind that are commonly used at birthday parties can be quite dangerous, as they can block of children's airways and cause emergencies. For the most part, this sort of problem becomes obvious quite abruptly, as a child will begin to cough or have other problems shortly after ingesting the foreign material. It can be sucked into the lungs if the piece is small enough, or it can be something that is life threatening. Usually, if there is a concern for something abnormal being swallowed, it's important to speak with a doctor
soon. If the story is concerning, and there are any concerning findings on physical exam, it is usually a reason to verify that there is nothing stuck where it shouldn't be. If something is swallowed all the way into the GI tract, it is often able to be expelled without difficulty (with a few exceptions, such as batteries, that are quite serious). If it goes into the airway, it needs to be addressed and may need to be removed. Please speak with your doctor about your question and concerns, and go to the emergency department if necessary.