WASHINGTON (Reuters) - Tens of thousands of children unaccompanied by parents or relatives are flooding across the southern U.S. border illegally, forcing the Obama administration and Congress to grapple with both a humanitarian crisis and a budget dilemma. An estimated 60,000 such children will pour into the United States this year, according to the administration, up from about 6,000 in 2011. Now, Washington is trying to figure out how to pay for their food, housing and transportation once they are taken into custody. The flow is expected to grow. The number of unaccompanied, undocumented immigrants who are under 18 will...