Taken literally, the text of Jeremiah 19:9 does say that God would cause the people of Judah (not Israel, see 19:3) to engage in cannibalism, thereby making them break God's own law. However, the prophecy need not be taken as saying that God literally would do this. Rather, it may be seen as a warning to the kings and people of Judah to change their ways. Consider what the same prophet says in the previous chapter:
7 At one moment I might speak concerning a nation or concerning a
kingdom to uproot, to pull down, or to destroy it; 8 if that nation
against which I have spoken turns from its evil, I will relent
concerning the calamity I planned to bring on it. (18:7-9)
With that understanding Jeremiah 19:6 should not be taken as determined by God or as absolute. God's heart in issuing some prophecies is to cause a change in the nation's attitude. It is the people who choose, and God is only too happy if an outcome results that negates the prophecy. In other words, the "if-then" of the previous chapter is implied in the warning of chapter 19. One also thinks of the Book of Jonah, where God instructed the prophet to foretell the destruction of Nineveh in 40 days and the prophet pouted when his words did not come true. God, thankfully, does not always share the attitude of his instruments.
The OP asks "Is He breaking His own law?" The answer his no, because it is not literally God who causes the evil, but people; and the outcome would be different if the people and kings of Judah would repent. It also asks "Can God break His law?" On that question I would say (admittedly a personal opinion) that God does not do so; whether he can or not is too big a question for me.
Conclusion: God did not make people eat their own children. The prophecy was not meant literally in that sense, and it was also not absolute in its outcome. It was up to the people, not God, whether the warning would be fulfilled. He did not break his own law in this case.