WASHINGTON — From now on, children at a family shelter in southeast Washington, D.C., may get a kick out of the fact that they are swinging and sliding where two of the country’s most famous children once played. On Monday, as part of their move from the White House, President Obama and first lady Michelle Obama personally donated to the shelter the play set they surprised their daughters with eight years ago, when the girls were 10 and 7. Most of the families at the Jobs Have Priority Naylor Road Family Shelter are headed by young African-American women, according to the shelter’s website.