When it comes to FP, I suggest Haskell without second thought, because it's pure and forces children to think functionally. Haskell's clean syntax also makes it easier for children to read and write code.
On a side note, Haskell also resembles much of mathematical notations kids learn in school anyway, more so than any other language (guards, functions, etc.) Teaching it would not only help kids with programming, but also would serve as a tool to implement math lessons on a computer.