If you were not aware of this, button batteries are incredibly dangerous to children. There is a misunderstanding about these miniature killers. It is not the contents leaking that are dangerous, it is the electrical current which forms corrosive chemicals outside of the battery. Button batteries have become more powerful, in order to meet the demands of today’s toys and gadgets. When swallowed, the current may burn a hole in the gut (usually the oesophagus) and bleeding can be fatal. A swallowed button battery needs to be located as an emergency in case it is stuck, as these carry the highest risk.
I find that many parents don’t know about this, so I often mention it when a child has swallowed something concerning but less harmful, like a diamond ring.