President Barack Obama has described the growing influx of unaccompanied children migrating to the United States as an "urgent humanitarian situation".
Latest estimates say as many as 60,000 children, mostly from Central America and Mexico, will enter the US illegally this year.
The White House has asked Congress for an extra $1.4bn (£836m) to cope with the situation.
It said the US would temporarily house the children at two military bases.
More than 90% of children caught trying to cross the Mexico-US border without parents come from Honduras, Guatemala and El Salvador.
Many are escaping domestic abuse or the violent gangs that prey on them.
Others are driven by poverty in their home countries.
In the US, many of the children hope to reunite with a parent or other relatives.
The director of the White House Domestic Policy Council, Cecilia Munoz, said the number of children travelling alone had been increasing since 2009.
She said the number of girls and of children under 13 had been on the rise, too.
"All these things are contributing to the sense of urgency," she said.
The US government says the number of unaccompanied child migrants could grow to nearly 130,000 next year.
Between 2008 and 2011 the number of children in the custody of the Office of Refugee Resettlement fluctuated between 6,000 and 7,500 per year.
But 13,625 unaccompanied children were apprehended by border agents in 2012 and almost double that number - 24,668 - in 2013.
Many are unaware of the dangers they face when travelling to the US, including sexual assaults, forced labour and hunger.