Their hair is natural. It looks darker from a few years ago, it was much blonder then. It's normal for natural blonds to go a mousy colour and the roots being darker when you have long hair that's being naturally bleached by sunlight.
Agree, my boys are the same, the ends go very blonde in the sun. Leo particularly has a very blonde (white) streak in the front, and has had since she was a baby. Doesn't Leti have the same? It's called Poliosis and can be hereditary.