Coin cell batteries can kill toddlers within 48-72 hours if swallowed. Even if they don't kill, they can cause terrible internal burns when the gut shorts the terminals.

If there is any suspicion of swallowing a coin cell, go direct to hospital. Do not give the child anything to eat or drink until given the all clear.

http://www.bbc.co.uk/news/uk-england-manchester-29610570