This is really oversimplified; the generations are not defined by who birthed who. Just looking at the dates most commonly attributed to these generations, it is clear that at least half of Millennials were born to Boomers and not Gen Xers.
When Millennials started being born in 1981, the oldest Gen Xers were only 16. At the time, the average American woman wasn’t even starting to have kids until she was 23, so you’re looking at 1988 as the first real set of kids born to Gen X mothers.
The last Millennials were born in 2000, at which time the average American woman wasn’t starting to have kids until 25, placing them at 1975 (in the latter half of Gen X).
If the average family has two kids and the average gap is 2.5 years, then the average mother in 2000 was 26 (while the average father is two years older). Going backwards, it looks like 1991 (the very middle of Millennials) is when Gen Xers started having the majority of children in the US.
Though one could argue that this was really a couple years later because these generational descriptions tend to be about white, middle class Americans, who tend to have children a couple years later than the average American.