This is a textual discrepancy. Furthermore, the number is irrealistically high. Chronicles was written between 350-300 BCE, and according to Israeli archaeologists Magen Broshi and Yigal Shiloh, the population of ancient Palestine as a whole did not exceed one million in that era. Furthermore, not all of the population would have been Jewish.
The male and female birth rates in humans are identical - so that leaves 500.000 men of all ages and religions at best.
Today's percentage of children in the world population is around 25%. It would have been much higher in Hellenistic Palestine. With life expectancies of twenty to thirty, women would have to give birth to between 4.5 and 6.5 children to maintain replacement levels. Given the violent nature of the period (Alexandrian conquests), however, the birth rate would have needed to be higher than that baseline, at around 6 to 9 children per woman.
The Torah dictates a minimal age of 20 for military service (Num. 1:3, and Rashi and Naḥmanides). We can extrapolate from reliable data from a somewhat later age (the Roman Empire), taking into consideration that life expectancy would in comparison be lower:
We can deduce that at least around 275.000 males would have been 10 or younger. Assuming a "conscription" age of 20, that would leave at best 175.000 for the entire region, again still including all religions and not just the Jewish population. We know that there was a considerable Samaritan, Egyptian, Greek (Ptolemaic and Alexandrian Empires) and Phenician presence around 350 BCE.
Given that Judah comprises 30% of the Ancient Palestinian geographic area with cities, 50.000 Jewish men aged at least 20 years would be more realistic for that era. Perhaps not surprisingly, the difference is a factor of ten. Historical philologists have encountered the same systematic rate of exxageration in Jewish folk tales (Eliach 1982; Mintz 1968:356-79.)