One of the best things in the world that a writer can have is someone who is a great editor and critiquer. I am super lucky because I have two. My friend K is also a children's writer, so she 'gets' what a children's novel needs. She also understands stuff like the level of complexity - you can have a lot more in a children's novel than you think - what you can't be is obscure.
She has just done a critique for me on a novel that I've been working on for about four years (on and off, because I have to have time out between drafts). She saw an earlier draft, which she really liked, despite its problems. I had changed a lot this time around, including point of view and a lot of the plot, and I wondered what she'd think of the new version.
Her insightful comments were terrific, and I love it when someone is really picky. Even little things that jar can pull the reader out of the story, and it's hard to pick them up yourself. I plan to return the favour soon.
My other friend T is also a great editor. She's picky in a different way. She doesn't write or even read children's novels, so she critiques from a different perspective. She's the person I go to when I know something is wrong but I can't figure out what it is. Through discussion, we often succeed in identifying where the problem lies. She is also merciless.
Now, neither of these two are going fix everything, and neither should they. Ultimately it's still my job to get the manuscript to the best I can before handing it over. They're not there to fix my punctuation and spelling, although they might pick up occasional awkward sentences. The grammar stuff is MY job, and this is something I try to drum into students.
An editor picking up an unsolicited manuscript is not going to bother reading something with five or ten mistakes on every page. There are always some people who honestly believe that the brilliance of their writing will overcome the obvious fact that they don't know how to punctuate a sentence so it's readable. The sad truth is: if you can't construct a good sentence, your writing is not going to be brilliant.