The disc battery in that hearing aid poses the most danger to your dogs. Here's a quick summary of this kind of ingestion:
Potassium hydroxide, sodium hydroxide, cadmium, lithium, mercuric oxide, and zinc.
Ingestion of button-cell batteries can lead to esophageal erosions due to the release of potassium hydroxide or sodium hydroxide. Batteries in the stomach can cause gastric erosions and ulceration. These erosions may develop within 12 hours, with the area in contact with the battery suffering the greatest damage.
Lack of appetite, vomiting, abdominal pain, and drooling.
Seek veterinary attention.
General treatment: The button-cell battery may be removed with an endoscope. This allows for an exam of the esophagus and stomach at the same time. Abdominal surgery may need to be performed if endoscopy is unsuccessful at retrieving the battery.
Supportive treatment: Secondary conditions such as erosions or ulcers are treated with medications and dietary changes. The pet is monitored for symptoms of mercury or heavy metal poisoning and treated if necessary.
Specific treatment: Unavailable
Good, if the button-cell battery is removed immediately.
I understand the dilemma of having to X-ray four dogs but that should be considered at this time. Please respond with further questions or concerns if you wish.