Matthew 2:16-18
16 Herod was furious when he learned that the wise men had outwitted him. He sent soldiers to kill all the boys in and around Bethlehem who were two years old and under, because the wise men had told him the star first appeared to them about two years earlier. 17 Herod's brutal action fulfilled the prophecy of Jeremiah: 18 "A cry of anguish is heard in Ramah – weeping and mourning unrestrained. Rachel weeps for her children, refusing to be comforted – for they are dead."

vs the original OT Jermemiah 31:15

Jeremiah 31:10-18
31:10
"Listen to this message from the LORD, you nations of the world; proclaim it in distant coastlands: The LORD, who scattered his people, will gather them together and watch over them as a shepherd does his flock.

31:11
For the LORD has redeemed Israel from those too strong for them.

31:12
They will come home and sing songs of joy on the heights of Jerusalem. They will be radiant because of the many gifts the LORD has given them – the good crops of wheat, wine, and oil, and the healthy flocks and herds. Their life will be like a watered garden, and all their sorrows will be gone.

31:13
The young women will dance for joy, and the men – old and young – will join in the celebration. I will turn their mourning into joy. I will comfort them and exchange their sorrow for rejoicing.

31:14
I will supply the priests with an abundance of offerings. I will satisfy my people with my bounty. I, the LORD, have spoken!"

31:15
This is what the LORD says: "A cry of anguish is heard in Ramah – mourning and weeping unrestrained. Rachel weeps for her children, refusing to be comforted – for her children are dead."

31:16
But now the LORD says, "Do not weep any longer, for I will reward you. Your children will come back to you from the distant land of the enemy.

31:17
There is hope for your future," says the LORD. "Your children will come again to their own land.

31:18
I have heard Israel saying, 'You disciplined me severely, but I deserved it. I was like a calf that needed to be trained for the yoke and plow. Turn me again to you and restore me, for you alone are the LORD my God.

Again we see that this prophecy has nothing whatsoever to do with what the New Testament was referencing. The prophecy in Jeremiah is just another 'Jehovah kicks Israel around but there will be a utopia' prophecy. The 'children' referred to in 31:15 are (in context) not true children but soldiers who were on a campaign in a foreign land.

Does it make sense that this has anything to do with the 'slaughter of the innocence'? Did the children come back 'from the distant land of the enemy'? Did they 'come again to their own land'. Does it make sense to say "For the LORD has redeemed Israel from those too strong for them." when Israel gets crushed by Rome 70 years later?

A real prophecy would say something like 'Sarah (the mother of Israelis) mourns for her children. The devil seeks to slay the Lord made flesh but his sword finds only the blood of the innocent.'