Oh this looks like a lucky escape. The button battery might have caused serious damage - internal burns in oesophagus, stomach and intestines. Even perforation!
If the child is active and playful, no tummy pain and no diarrhea or vomiting it should be fine. But you can do the following to help the baby protect against any corrosion caused accidentally-
Give 2 tsf of honey every 10 min - upto 6 doses - the weak acid of honey neutralises the alkalinity of button battery
( jam given in less than 1 year old - the child is 2 years as I can see, so honey is great)
Next Steps
If any symptoms persist - Green stool/ stomach pain/ vomitings please visit nearest hospital emergency
Health Tips
Get a stool test for routine and occult blood
Buy toys and products with child safe policies. Button batteries, any other batteries to be kept locked, beyond reach of the baby