Police were called to an incident at Tiny Toes nursery in Cheadle Hulme, Greater Manchester, on Monday.
The girl was taken to hospital where she later died.
The two women, aged 35 and 34, were held on suspicion of gross negligence manslaughter, Greater Manchester Police (GMP) said.
The force described the death as “heartbreaking”.
Det Supt Phil Duffy said: “Our thoughts are very much with baby girl’s family who are understandably shocked and devastated.
“We’re doing everything we can to get them the answers they deserve.
“Officers are continuing to work to establish the full facts and ascertain the circumstances.”
A spokesman for Ofsted said the nursery, on Mellor Road, had been temporarily closed and declined to comment further.