The law is clear: no they cannot. From Andrew P. Napolitano at lewrockwell.com:
Last weekend, President Donald Trump argued that those foreigners who enter the United States unlawfully should simply be taken to the border, escorted across it and let go. According to the president, this would save precious government resources, avoid the business of separating children from their parents and free up the Border Patrol and other federal assets to do their jobs.
He is undoubtedly correct on the beneficial consequences to the government of forced deportation without due process. Yet deportation without a trial is profoundly unconstitutional.
Here is the back story.
The nation has been torn apart by the images of immigrant children — some are babies — being forcibly separated from their parents by U.S. immigration authorities, who were getting orders from the Trump administration, which was misreading federal law so as to require the separation.
The government has essentially taken the position that those physically present in the U.S. illegally have few constitutional rights and thus family members who arrive together can be separated, no matter the psychological or physical consequences. This forced separation is not novel to the Trump administration, but its massive scale in the present toxic national political environment has painfully brought it to our collective conscience.
The forced separation by the government of children from their parents without a trial when neither is a danger to the other is child abuse or kidnapping or both. When federal authorities engage in such morally repellant behavior — whether as a negotiating technique to bring the president’s political adversaries to the bargaining table or to coerce the immigrants to go home — it exposes them to state prosecution because of the acute and long-term harm they have caused to the children.
After a tidal wave of public opinion against this practice finally resonated in the White House, President Trump signed an executive order last week that permitted, but did not require, immigration authorities to reunite the children and their parents. Then, in the wake of a slow reunification — some of the children had been sent from Texas to New York while their parents were kept in Texas — the president uttered his exasperation regarding due process.
To continue reading: Can Immigrants Be Deported Without a Trial?