It is true that children's books are often written by a team. The reason is that often a writer is not a good illustrator and an illustrator is not a good writer. Publishers often, not always, do the matchmaking between the written material and the illustrated material. This isn't a hard and fast rule, however. There are many books, including the gorgeous "Stellaluna," that were illustrated and written by one person. I'd say if you have an idea and can both write and illustrate it, go for it. I would suggest you might look at the publishers' guidelines. They may want the illustrations submitted separately from the text (in other words...they may not want it perfectly laid out on the initial submittal). From a library point of view, there is a huge thirst for good children's books out there. It's a wonderful thing to see parents checking out 20-30 illustrated books a week so they can read to their children.