There is plenty of criticism of the NKJV - how it was first understood to be an 'updating of archaic KJV language'...to how it ended up perverting KJV text. On this topic, however, they have like-minded company.
2 Sam. 12:31 - NKJV - And he brought out the people who were in it, and put them to work with saws and iron picks and iron axes, and made them cross over to the brick works. So he did to all the cities of the people of Ammon. Then David and all the people returned to Jerusalem.
1 Chron. 20:3 - NKJV - And he brought out the people who were in it, and [a]put them to work with saws, with iron picks, and with axes. So David did to all the cities of the people of Ammon. Then David and all the people returned to Jerusalem. ([a] - LXX cut them with)
After the killing of the people in 2 Sam. 12:31, the Hebrew reads: ...the brzl/iron u-eobir/he-caused-to-pass auth-m/them b-mlkn/in-MLKN....
Is MLKN actually a plural for the fires of Molech, the smelting furnaces for their iron foundaries?
Knowing what we do about Yahweh forbidding the "passing through the fire to Molech" (Lev. 18:21), it seems plausible that this is what David did to the inhabitants of the Ammonite cities - after killing them first. Perhaps it's another example of where the scribes choose to 'veil' incidents where revered personages are 'behaving badly' in their eyes.
It seems highly unlikely (read: downright impossible), to me, that Yahweh would have permitted the Ammonite people of multiple cities to be brought into the borders of Israel. Or to have Israelites become 'supervisors' over them doing iron works in their own cities.
There is an interesting article on JSTOR. I get the free 100 reads per month, so don't know if this link will be viewable to others:
O'Ceallaigh, G. C. “And So David Did to All the Cities of Ammon.” Vetus Testamentum, vol. 12, no. 2, 1962, pp. 179–189. JSTOR, www.jstor.org/stable/1516445