Children can easily ingest objects 2cm in diameter, and batteries that size can be especially dangerous because of how they lodge in the esophagus.
Citing Eck, Langham says, "Clinical outcomes can be determined by assessing the diameter of the button battery, as 90% of all major or fatal outcomes are associated with lithium batteries of 20 to 25 mm in diameter"
Those statistics are over a decade old, they also clearly summarise the danger of 20mm sized batteries.
'RESULTS: All 3 data sets signal worsening outcomes, with a 6.7-fold increase in the percentage of button battery ingestions with major or fatal outcomes from 1985 to 2009 (National Poison Data System). Ingestions of 20- to 25-mm-diameter cells increased from 1% to 18% of ingested button batteries (1990–2008), paralleling the rise in lithium-cell ingestions (1.3% to 24%). Outcomes were significantly worse for large-diameter lithium cells (≥20 mm) and children who were younger than 4 years. The 20-mm lithium cell was implicated in most severe outcomes.'