The two children in the large prisoner swap yesterday were actually children of two Russian sleeper agents. The children were unaware they were Russian and were informed on the plane to Moscow according to the Kremlin.
They don't speak Russian, so Putin greeted them in Spanish.
Their parents, Anna and Artem Dultsev, were posing as Argentinians in Slovenia, but were actually Russian spies.
Such Russian sleeper agents have been in the US as well.
On a related note, everyone should watch the phenomenal TV series "The Americans". It is one of my favorite series.