This may sound like a strange topic, but it's an insiders' trick that I -- as well as many other writers -- use to make researching new topics easier. And I don't mean just for researching children's books; I use it when writing books aimed at adults as well.
The trick is doing your initial research in children's reference books. And yes, you read that correctly.
I certainly understand if you wonder why. It's not the first idea that comes to many writers. (HINT: That's why it's an "insiders' trick.")
When you first begin researching a new subject, the amount of info you have to sift through can be mind-numbing. For example, I'm working on a humorous adult fantasy story with an Oriental flair. Have you ever looked at any of the standard reference books on ancient China? They're HUGE! Each one cover different material, and it's possible that none of them will give me what I'm looking for. Some of them sound promising but end up focusing on topics more suitable for academics than fantasy readers.
So.... I ignored the adult reference section in the library. Instead, I wandered over to the children's section to see what they have on Chinese history. I ended up with a couple of titles -- The Ancient Chinese by Virginia Schomp and The Han Dynasty by Myra Immell. Neither of these books is more than 100 pages, yet both cover their topics fairly well and are easy to digest quickly.
Now don't get me wrong. There's a good chance that neither of these books will give me all the info I need. (Then again, they might. I just need some Oriental flavor, as my story is fantasy and not intended to be historically accurate.) But even if they don't, they give me a quick overview of the topic. I can identify things that interest me, then use that info to navigate my way through those thick adult reference materials. That way, I find the useful info much more quickly so I can get back to writing.
In addition, sometimes I uncover little facts that really add to a story. For example, I quickly learned that rice was the main crop in southern China, not the entire country. (In northern China, farmers tended to grow wheat and millet because of the weather differences.) Those are the kind of details that make a story interesting -- that's why it's in the children's book -- yet I might have gone through several large adult tomes and never found it.
Furthermore, these children's books provide suggestions of other potential references, both in books and online. And because these are children's books, and children aren't interested in things like the psychological impact of imperial rule on the downtrodden peasants, they don't bore me with musings about things that won't make a good story.
You won't always find a children's book that can get you started, but you will find one often enough to make it worth your while. So don't ignore this "shortcut" when you need some quick research. Children's writers tend to be fairly intelligent and helpful people, you know. ;-)
The book cover image came from this page at Scholastic.com.