Well, overfeeding has met the CPS's threshold for prosecuting as a crime, or they wouldn't have charged her for (child G):
Count 7: Attempted murder (air embolus & excessive milk)
Count 8: Attempted murder (air embolus & excessive milk)
Count 9: Attempted murder (air embolus & excessive milk)
It would have just been air embolus.
The baby's milestone birthday is to discredit the defence's theory that the baby died simply because they were so premature. Also baby is the victim and the prosecution needs to talk through all the details of the day they were attacked - which just happened to include a milestone birthday.
As an e.g - With Covid, 14 days (or whatever it was) was the end of self isolation i.e you were considered much healthier and ready to return to the world at that milestone. Now if you suddenly died on day 14, Covid wouldn't automatically be considered the killer because you passed the threshold of it killing you.
The 14 days is like the 100 days. 100 days is milestone for premature babies that shows their chances of survival now are much higher.
Baby G was given a 5% chance of survival at birth - it's reasonable to think that at 100 days this was much much much higher.