Before I pitch a picture book manuscript, I must answer YES to all of these questions.
1. Would my kids want to read it?
This may sound like a no-brainer, but there are a lot of books out there (and even more DOA manuscripts) that are mildly interesting to adults but not to kids. I pitch my story ideas to my kids, and if they say "Oooh, can we read that?" then the idea is worth pursuing.
2. Would I want to read it?
If this story were published, and my child loved it and went to grab it from the shelf every night for a week, would I happily read it again? Or would I groan and convince them to pick something else, anything else? (Come on, you know you've done it: "Now, I know you LOVE it when Brother and Sister Bear go back to school AGAIN, but can we try something else tonight? FOR THE LOVE OF GOD?")
3. Does every word count?
I finished a manuscript last March that I worked on for several weeks and edited down to a respectable 700 words. I thought it was finished. I just picked it up a few weeks ago and cut it down to 376. I know now, for sure, that every word in that manuscript counts. There is nothing superfluous.
4. Did my critique partner read it? Does she like it?
I trust my writing partner very, very much, and if she hasn't worked on it, it ain't ready.
5. Does it have a LOT of illustrative moments?
The story cannot get by on the strength of its words. The best picture books being printed today have a unique interplay of story and words. Unfortunately, I cannot draw one iota. So when I write, I have to provide areas that would allow an illustrator's imagination to run wild.
6. Does it have a good narrative arc?
If the story doesn't have an interesting problem, build-up to climax and then resolution, forget it. This is really, really hard to do in a few hundred words. It takes a lot of practice and a LOT of reading. When I read books to my kids that don't have a good narrative arc, it makes me absolutely crazy. No one cares about an interesting character if he doesn't do anything interesting.
7. Does it have a surprise ending?
This isn't a "must" for some writers, but it is for me. I absolutely LOVE the final page turn in a picture book to find one last joke, hook or surprise. A great example is the final page in "Never Babysit the Hippopotamuses!" by Doug Johnson. The last page spread says "So. Never babysit the hippopotamuses. Unless, of course, your only other choice is to babysit their neighbors," ... page turn ... "the MONKEYS." That kind of ending gets kids (and adults) every time.
I have several manuscripts languishing right now because they answered NO to just one of these questions. One story is well-loved by my family and friends, but I know it lacks a narrative arc -- it's just too episodic to be interesting to a publisher. Another is a great story without enough opportunity for illustration. It could potentially be turned into a short story, but it's just not a picture book.
If a story answers a YES to all of these questions, I feel really good about it. If I send it out and it gets rejected a million times over, honestly, I still feel really good about it. That sounds a bit too Pollyanna, but it's true. I know I gave it my absolute, best shot. And then it's time to get back to writing some more.
p.s. I didn't include one thing because it may seem too obvious. But it's a no-brainer. No spelling errors. No grammatical errors. DON'T BE A MORON. The people reading your manuscripts read for a LIVING. If they see an error in a few hundred words, they probably think you are dumb. Don't do it.