Writing picture books might seem like the easiest writing job ever but not so! Think about those little 'ankle biters' and how they have to be entertained every minute. If you can't think like a baby then you can't write like a baby and your picture books will not do well. You are talking writing that needs to clear, concise and baby friendly. If you are accustomed to writing books with thousands of words you will need to think and write in the hundreds.
Writing a picture book might not take as much time to actually write but the research and development process is longer than you would think. Go to the library and see which picture books are the most popular. You can tell because they are the ones falling apart at the seams and they have little bite marks along the cover's edge. Think simple but precise. Make the words flow lyrically. They should help conjure up pictures in your mind and also sound good being spoken out loud.
Little minds are very, very bright so don't even think about writing down to them. They want to be entertained with their picture books but they also want to soak up sights and sounds like a sponge. Choose verbs that sparkle and skitter across the page. Use adjectives sparingly like a dieter uses butter, just enough to add taste and flavor but without all the fat. Keep a thesaurus handy and don't be afraid to use 'big' words. These little guys are learning and learning quickly. You would be amazed at the things they say when they start to talk and their picture books need to be able to keep up.
Descriptions are nice but these kids want picture books that move them-picture books that make them feel silly, happy or even sad. If you are going to try and rhyme make sure you have your meter down tight. If it doesn't flow it will be dead in the water. If it doesn't come naturally don't try and force it. It's like someone who sings off key no matter how hard they try it still hurts your ears.
If you are also an illustrator you might be able to do your own artwork, but not always. Picture book writers should write with the end product in mind. Visualize what the illustrations or artwork might look like and write to match.
When you are done try out your ideas on the people in your writer's organization, if you are a member of one. If not consider joining one. Many offer critique groups and online message boards that can help you by giving their opinion on your book. If you have access to a children's playgroup or preschool class or even little playmates of your children get their opinion. Older children make good critics with the ability to even be constructive in their comments!