If the mother is new she may have birthed prematurely… it happens sometimes.
Guppies are quite tolerant of temps but a heater may help. It is odd that there’s 0 nitrates though, it can happen in heavily planted tank but normally that’s the time when I add some ferts. Not sure if more food was needed, especially in a planted tank with no other fish, I personally only feed guppy babies 2x per day on average and they do fine.