Jamieson, Fausset and Brown on Ezekiel 16:20-21

20-21. sons and ... daughters borne unto me--Though "thy children," yet they belong "unto Me," rather than to thee, for they were born under the immutable covenant with Israel, which even Israel's sin could not set aside, and they have received the sign of adoption as Mine, namely, circumcision. This aggravates the guilt of sacrificing them to Molech.

to be devoured--not merely to pass through the fire, as sometimes children were made to do (Le 18:21) without hurt, but to pass through so as to be made the food of the flame in honor of idols (see on Is 57:5; Jr 7:31; Jr 19:5; Jr 32:35).

Is this of thy whoredoms a small matter, that thou hast slain my children--rather, "Were thy whoredoms a small matter (that is, not enough, but) that thou hast slain (that is, must also slay)," &c. As if thy unchastity was not enough, thou hast added this unnatural and sacrilegious cruelty (Mi 6:7).