Firstly, we don't know the children in this caravan are the actual children of those who have them in their custody.
Secondly, it's a political ploy old as the institution itself.
What fit parent travels with a sick child in order to try to break into a country?
Kids getting sick on the trail is a plus. If they die while being repelled at our border, it makes the U.S look heartless. That's a plus for the Democrats that say, let everybody in!
But do you really want a kid with the flu to come into this country as they would have before? Risking infection of those north of the border with their virus?
Keep in mind our border is also the gateway for cartel traffic, the Satanist worshiping gang, MS-13, and sex traffickers. Which also goes back to the children in the caravan not necessarily being the offspring of the adults that have them in their custody.