The most common problems, of course, are making the child sound younger or older than they are supposed to be. And I can see why this is so tough. In fact, some of the problem may lay with the reader (me). There is such a wide range of development in kids that the author may be using something within the acceptable range but it doesn't sound right to me if it isn't the same as my own kids would act/speak.
There are probably a lot of 5 year olds that would say they "writed their name." My 3 year old would say that, but my 5 year old would not. So if the author tosses that sentence in for their 5 year old narrator, it's a turn off for me. It doesn't seem authentic.
Plus it's just tough to read an entire novel told from a child's perspective because the language is so different than what I am used to. It can be charming sometimes, or interesting, or eye-opening. But it can also be annoying and simplistic.
I'm thinking about this right now because I just started a novel that has a 5 year old narrator, and I am not sure if I am going to like it as much as I thought I would. I read a great review about Room which made me intrigued enough to purchase it from amazon. But already in the first chapter I have had some trouble with the "voice" of the narrator.
A while back I tried the book The Sweetness at the Bottom of the Pie which features an eleven year old girl as narrator. I only made it a few chapters in before giving up. That "voice" just wasn't appealing or able to hold my interest.
What has your experience been with this?