Why wasn't the Three Eyed Raven one of the Children of the Forest ?
Rather than a man? The Weirwood trees and their associated woo-woo was a CotF thing wasn't it? So why would the High Priest be one of the men that the CotF were at war with?
So that Bran can become TER 2.0, obviously.
shareI was never a fan of the 'Bran the Flying Ent Avenger' thing. At least on the show he's not going to turn into a tree. And as for the magical trees, wtf is that all about anyway? Monkey memory mumbo jumbo that's what! But keep the CotF and their Holy Hand Grenades, that was cool.
The magic trees do seem a little bizarre. But you'll notice that every religion mentioned in the show (at least so far) has genuine power for its believers. The Lord of Light and the Drowned God resurrect people from the dead. The Old Gods have magic trees that green seers can access. The Faith of the Seven had the Faith Militant (you could also argue that Arya's assassination of Walder Frey was divine justice by the Seven as well). The Many-Faced God gives its followers the ability to turn into others. And the Great Stallion can hatch dragons.
Also, remember that, in a high fantasy world, religion can be whatever the creator wants it to be. If he wants it to be similar to a faith practiced by many people here in the world today, it will be. If he wants to be something completely bizarre and different from anything practiced by any substantial group of people today, it can be that, too. The weirwood trees are just part of Martin's worldbuilding. Personally, I find them fascinating, but you don't have to.
(But yeah, those holy hand grenades are pretty awesome.)