You seem not to understand "emergency." It doesn't matter if there was one emergency or a dozen.

It doesn't call for debate on whether to return to the gate. It means get medical personnel, in this case EMTs, there STAT. This was an infant. Infants don't have the ability to control their own body temperature to the degree older children and adults do. The consequences of heat stroke can be profound, including permanent brain damage and death. 

If you don't already work for United, your post shows you have the cavalier attitude towards customers that United obviously cultivates.