Is your baby having fever? And othe complents?
It is only sweat rash. In baby their sweat glands get blocked due to sweating. It happens on all sweaty parts of body.
Clan and dry all body parts.do not use any powder, utbarn , moisturizer and any oil.it will get worsen if you use any of them.
Use cotton clothes to baby. Only 1 layer of clothing good for baby in summer. After feeding clean your baby with only water.
Maintain your room temperature between 26-28 degree. For that you can use AC, Cooler ,Fan .just make sure the direct breez not toches to your baby.
Next Steps
if you want you can use Calamine lotion on affected part.
if required do consult.