Children's books vary greatly depending on what age group you're writing for. They can range from picture books, with a few words, for very young children to short chapter books, with no or few illustrations, for pre-teens. When you have decided the age group you want to write for, research popular books for kids of that age to find out what they like. You could even volunteer at a school or pre-school to learn firsthand about the children you'll be writing for.
An important part of any book is an engaging plot. For younger children you should stick to very black and white plots, where good always triumphs over evil, all problems are solved after a bit of hard work and there are no complex moral dilemmas. Older children can handle more complex plots and situations, but you should still avoid writing anything too dark in a children's story. If you want to delve into this territory, try writing young adult books.
Believable characters are a must in a children's book. Your book's protagonist should also be likable -- children's fiction is no place for an anti-hero. Spend some time around children to learn how they act and talk amongst their peers, as well as how they interact with grownups. Try to think back to your childhood to remember how you felt. It is all right for ordinary children to have extraordinary powers in children's books, especially those for younger kids.
It's all very well to know what you want to write, but in order to create a children's book you have to have the time to actually sit down and write it. If you're writing full-time, treat it like a regular job and set yourself working hours. If you're writing part-time, make sure you manage your time effectively to make the most of your writing time. You should have an office or other quiet place to work, to avoid distractions.