Jamieson, Fausset and Brown on Ezekiel 20:31

31. through the fire--As "the fire" is omitted in Eze 20:26, Fairbairn represents the generation here referred to (namely, that of Ezekiel's day) as attaining the climax of guilt (see on Eze 20:26), in making their children pass through the fire, which that former generation did not. The reason, however, for the omission of "the fire" in Eze 20:26 is, perhaps, that there it is implied the children only "passed through the fire" for purification, whereas here they are actually burnt to death before the idol; and therefore "the fire" is specified in the latter, not in the former case (compare 2Ki 3:27).