You are getting some great advice here OP - in fact, this thread should probably be pinned! - but, on rereading your first post, it seems as if you haven't even decided what kind of book you want to do ('possibly a children's book'). That seems like a recipe for disaster.
If you do want to write one for kids, here's another honest truth: I write full-time. You can go into pretty much any supermarket at any time and there will be one of my books on the shelf. This isn't my hobby, it's my job. I have three kids and, over the years, have done everything from rhyming toddler books to full novels for them. I haven't let my agent have even a sniff of them in ten years, I just print them off for my lot because I know I'd be on a hiding to nothing.
If you do want to write a children's book and it's because you have children of your own, or in your family, honestly - do that. They'll get the pleasure of it. In fact, it can be even more special as no one else has the story they have - I remember one Xmas, doing a chapter a day for one of my sons and leaving it on his pillow every night. When they were younger, I used to print the text off on A4 sheets, and they would do the illustrations.
I do encourage people to follow their dreams if they have a story to tell, but with children's books, it really is another world out there!