1. Give them permission to hate it
If you are reading to kids, make sure that they know it is okay not to like what you are reading to them. Make sure that they understand that you want their honest feedback because it will make your writing better. Sometimes that's all it takes for them to tell you want they really think.
2. Reactions speak louder than words
Do they laugh at the right parts? Do they groan when you want them too? Kids can control what they say (for the most part) but their reactions come naturally. Make sure to watch and listen when you are reading. I always know when my words have fallen flat when my class starts fidgeting.
3. Be specific
Don't just ask, "So what do you think?" Ask questions about your characters and plot. The more specific you are, they more you'll know if they have been following you. Ask questions such as, "If there is one thing you would change about what I just read, what would it be?"
4. If you have to explain it, something's missing
As in any critique, make sure not to justify your work. Listen to what the kids have to say. If there is something they didn't understand, don't explain it. Think of ways to clarify the idea in your writing.
No matter what kids think of your writing, remember that information is for you, not for the agent or editor you query. For more query writing tips, make sure to check out this recent post by Chuck Sambuchino that includes advice from my agent, Linda Epstein.