One thing to note about children's IQ is that, contrary to intuition, environmental factors tend to play a smaller overall role as the child mature into adult. Instead, their genetics seem to play a larger role.

As the children mature, the heritability of their IQ grows from roughly 0.45 to 0.8 (similar to height) [1]. The fact that the age of the child at the time of the IQ test is such a large factor is often not accounted for in research, which creates inconsistent results. Ideally, breastfeeding research would compare the IQ at adult age of children that were breastfed for different durations, normalized against their parents IQ.

https://en.wikipedia.org/wiki/Heritability_of_IQ#:~:text=Ear....