This is a tough one. As my older son is 8 at the moment, and quite literate, I'm often tempted to see if he'd read something for me aimed at younger audiences. On the other hand, while he's quite happy to read any words you stick in front of him, getting useful feedback is a challenge. He's not unwilling to talk, just, well, eight. Easily distracted, and more likely to try to do a mash up of whatever I've written and Transformers/Power Rangers/Pokemon, whatever his latest obsession is.
I'd aim for a bit older kids, if you could, as I think you're more likely to get cogent feedback. Also, I'd try to focus their feedback as best you can. Kids aren't going to have experience giving feedback. Make it easy on them. Like this: Put stars next to any sentences that you loved. A smiley face next to anything that made you laugh. Circle anything you don't understand (words, phrases, paragraphs, etc.) Put an X by anything you didn't like. You could also try direct questions about things like: Which character did you like? Which character would you want to be friends with? Which character would you want to be? What you do if you were <character X>? You can probably do similar questions with setting elements too. For example, if you were writing Harry Potter, you could ask: Which house would you want to be sorted into? Etc. But I wouldn't go crazy with the questions. Keeping it short will probably be more useful.
Good luck!