At present, residents of diverse Latin American nations are sending children across the borders into the United States. Current press articles are reporting on this controversial issue with terms like "flooding" and "hungry, innocent children", and an inestimable number are coming without their parents.
This question isn't about that current crisis, but it is an inquiry about parallels to it in the past.
Has there been a similar phenomenon in the past? How was it handled? How has it altered the populations of the countries involved?
It's definately not my area of expertise, but I've been looking at some letters and documents in an archive during a couple of university courses and one particular set fits well into your question. During the (Finnish) Winter War, Finnish children were sometimes sent over to Sweden to live with distant relatives. These children would be Finnish-speaking only or, if they were young enough, learn Swedish better than their native Finnish, and thus we have letters from them to their parents in Finland describing their living situation.
In this case the children who had surviving parents after the war were sent back to them when it was over. The letters don't tell the story of the rest since there was no one to send letters home to telling what happened next.
I could point you in a direction to find out more by yourself, but I'm afraid you'd need to be able to read 20th century Swedish handwriting since the documents aren't transcribed/translated (although possibly digital).
With some luck /u/vonadler might know more about this, although it may be a long shot. You could try sending a PM and hope for the best!