Analysis of information sources in references of the Wikipedia article "அம்மை விருந்து" in Tamil language version.
Complications of measles are most common among children younger than 5 years of age and adults 20 years of age and older.
Given the highly contagious nature of chicken pox, the thinking behind such events was, seeing as the child would probably contract it at some point anyway, why not catch it early and get it over with?