One of the most significant collections of Abraham Lincoln letters to be auctioned in decades includes his heartfelt reply to a group of youngsters who asked him to free America's "little slave children."

The 1864 letter is expected to fetch the highest price ever for a Lincoln manuscript.

"Please tell these little people I am very glad their young hearts are so full of just and generous sympathy," the president wrote in the letter to a woman who had mailed their petition from Concord, Mass.

Lincoln's hand-penned reply, estimated to be worth $3 million to $5 million before the April 3 auction at Sotheby's, could become the most expensive Lincoln letter ever sold.