I've heard from more than one expert that you should crowdsource book ideas before pursuing them. Figure out first whether anyone's interested in reading what you plan to write. That way, you'll know whether or not to invest your time in it.
But I say, "No!"
I'm a big fan of Denise Duffield-Thomas, a money mindset coach whose approach to business can be applied to many other pursuits (like book publishing). I'm going to apply her thoughts about pricing to this concept of crowdsourcing book ideas.
It's a waste of time and energy to ask people who aren't your target audience whether they like your idea. So asking family, friends, and strangers on the street will not likely help you gather useful information. This is true if you're targeting a very particular audience (like preschool age children), but it's also true no matter what your idea. People have widely varying tastes when it comes to books. It's not just about liking the idea; it's also about liking the style of your writing. Which they can't experience if you're just telling them about an idea.
2. Family, friends, and strangers on the street won't necessarily give you an honest answer. Some are going to spare your feelings and say they love an idea even if they don't. And some might respond negatively for reasons that aren't really valid or meaningful to you.
3 Describing a book idea can be a little like describing a dream. Even if you have a tight elevator pitch to shop around, you don't have a full grasp of your idea until you've fully explored - and written - it.
4. Can you really sample a large enough potential audience to determine whether your idea will succeed or fail? Doubtful! There are 7 billion people in the world. Surely some of them will like the idea and some will not. Those who don't aren't your audience, so there's no need to worry about them.
Don't crowdsource your idea. Write it! That's the best way to figure out if you love it (which I'd argue is most important) and the only way you can convince others to love it, too.