I agree with you, actually. I have a huge collection of folk/fairy tales from many lands, and many of my favourite books are children's books. And it's not nostalgia either. I read many of them first time as an adult. In lots of ways, children are less easy to fool. They aren't particularly taken in by fancy writing or fashion in writing. They just want a good story. One that sticks with them and makes them think about what happened in the story, and/or stimulates their own imagination. There's a reason so many of these stories have stood the test of time.
The adventures of medieval monks! Gripping read, I'm sure! I can just imagine them galloping past with their crosses and the Holy Book and whacking people's heads with it.
Just on that last bit, you seem to be saying that the author being male is an automatic strike against a female character being believable. If that's right, do you also consider female authors to be innately bad at writing convincing male characters?
Haha, nope, I do not believe that writers have to represent the characters they write about in order to write them believably.
I like to think the authors who actually believe that you can't write anything you yourself are not (eg. male author writing a female etc) are simply not very good writers. I mean, I sure as heck hope Nabokov wasn't a pedo when he wrote Lolita, or Pullman doesn't actually think he has a familiar who follows him around (well, pets, I guess - he may have a pet). I'm guessing crime writers have not regularly delved into murder and other forms of crime... and I'm pretty sure Tolkien was never a hobbit at any point in his life. Nor Rowling a wizard, or perhaps a wizardress? She might be an experienced muggle though. Anyway, point is, it's fiction. It's always gonna be about something made-up, and putting yourself into other people's shoes is part of what makes a good writer. That naturally lifts any boundaries regarding what the author must be in order to write a fitting representation of it in fiction. What you must do is research, without generalising too much, and there's always creative license you can use (y'know, that's just the way my character is)