When writing for children a popular method of getting started is to begin with an incident or happening. Any one situation, whether you've been told about it, or maybe heard about it on the radio, or maybe a chance sentence or bumping into someone may create a sudden unshakeable belief that this is a possibility for, one day the beginning of a book. Writing for children is, contrary to what many people believe, just as complex and challenging as writing for grown-ups, it's true to say that in a number of more so, so when you are writing for children, do choose your scenario or happening carefully as you need to gain, and keep a child's notoriously fickle attention.
In any case it may take years for you to begin writing for children, but the idea remains firmly fixed in your brain, and so does its potential for a good book. Here's an example for you to consider, it's the story of the manager of an orphanage who knew that many of the youngsters in her care had invented "real belonging mothers" belonging just to themselves. In some instances the mothers really existed, but many of these absent moms were made up by the children themselves to fill an aching void, because they didn't know who their real mothers were, they simply invented them.
When you are writing for children, you could possibly consider this scenario - store it away in your mind and give it time to take root. Possibly years later you may be able to use and develop it. Here's one possibility ....
Children in an orphanage create make believe mothers for themselves. Some invent warm, caring moms who love them deeply and only left them at the orphanage because they had no other way to carry on in a cold, hard world. Maybe other kids fantasise about heartless, uncaring women who simply abandoned them out of cruelty and spite - sounds a bit like a wicked stepmother - doesn't it? And we all know what wonderful tales have been woven around wicked stepmothers!
Of course not all situations will develop well. You do need to consider carefully what might be grow into a good storyline and which scenarios should be left well alone when writing for children. After all, you don't want to make your stories too scary! Remember those fantasy moms - those children needed make-believe mothers who would nuture rather than hurt them, and any writing for children should always leave the reader feeling warm and reassured after the end of the story.