John Gill Commentary Ezekiel 16:21

"that thou hast slain my children, and delivered them up, in causing them to pass through [the fire] unto them?" — Ezekiel 16:21 (ASV)

That thou has slain my children
By creation, as all born into the world are; and by national adoption, as all the Jewish children were; and particularly the firstborn were eminently his, and which are here designed, as Jarchi interprets it; for they were the children that were slain and sacrificed to Molech; see (Ezekiel 20:26) ; and delivered, them to cause them to pass through [the fire] for them ?
for the sake of idols, for the worship of them; this they did before they were slain; they first caused them to pass through between two fires, and so dedicated them to the idol, and then slew them; or slew them by burning them in the fire, or by putting them into the arms of the "idol", made burning hot.