If I were to write a childrens book, how would I write it?

[mummymonkey]

Define your target age group and then pick up some books from your local library that are aimed at that age. This will give you some ideas of how you should be writing. Once you know this you can work on what story you want to tell etc.
I'm a mum of two (3 and 5) who love books and reading and their favorite stories are told without words through pictures. Check out peter collington and oliver jeffers in particular.
You're either aiming to tell a story that hasn't been told before, or re-telling one in a different way. Otherwise no agent will take you on.
That is another point that you'll want to submit any work to a literary agent for them to present to publishing houses rather than going to them direct or you won't be taken seriously.
Anyway...once written, get in touch with your local preschool and see if you can go in and read your story to the children. As a preschool committee secretary I can tell you that you would be welcome as preschools are always looking for different things for the children to experience.
You can gauge how well your story might be received by the childrens response and this will give you a further reccomendation to a literary agent if you mention it in your cover note.
Hope that's of some help.