As I pointed out Wednesday when the Senate Homeland Security committee discussed the sudden new wave of illegal immigrant children, the right question is, “How do we stop the flow?”

America is a compassionate country. Naturally, we all want to treat the tens of thousands of children wading the Rio Grande humanely.

But true compassion means we have to think as well about millions more children in Central America whose parents are thinking right now about sending them on a terrible, dangerous trip. We need to prevent their parents from sending them. We must make it obvious that children who enter our country illegally will not get to stay.

We should do that by sending illegal immigrant children home immediately.

Families are subjecting their children on a incomprehensibly dangerous trip. Children as young as 5 are riding the roof of a freight train nicknamed “The Beast,” risking crippling injury or death. As the Los Angeles Times reported:

“The trip can take weeks or months; some get off the train along the way to beg or work, and those with children stop to rest and maybe pick up donated diapers or food before hopping another train.

“They ride on the roof, holding on for dear life, the luckier ones wedging themselves between jostling cars. Almost every one of them has had to pay bribes, either to Mexican police, immigration officials or gangs.

“In a twist, along some segments of the route, the notorious Zetas drug and extortion paramilitary force has been replaced by members of the equally ruthless Mara Salvatrucha gang, originally from Los Angeles and El Salvador, migrants said. They charge the migrants $100 at each stop, Honduran Jose Eduardo Calix said.

“ ‘If you don't pay, they try to throw you off the train,’ Calix, 30, said, adding that he had seen five people shot to death because they didn't have the money.”

Why would a family do this? It’s not just the endemic poverty and crime at home but the pull of the destination: No parent would send a child on a trip posing a risk of rape or death at the hands of criminal gangs unless she knew the child had a good chance at a great prize — being allowed to stay in a generous United States.

When the president stops deporting illegal immigrants and asks Congress for $3.7 billion in emergency spending to care for the wave of children who have already arrived, it tells desperate parents that the dangerous trip may well pay off. It tells Central American families that paying smugglers thousands of dollars and putting their children’s lives in danger is worth it.

If we have any decency, we must prevent that terrible decision.

There is no more humane thing we can do than telling parents that it’s pointless to subject their children to that horrifying journey. That means sending children home immediately.

There are millions of people in this world desperate for their children to get to America by any means. Few in America believe we should have totally open borders or would deny that because we are a nation of immigrants, our country is stronger and more vibrant. What we need is a functioning legal immigration system, and I will continue to work to achieve that important goal.

But first we need to address the root cause of this sad wave of children. That means having the compassion to see not only the children who have arrived but the ones who have yet to leave home.