In picture books text and illustration are merged together so well that one cannot live without the other without loosing a lot of its value. Of course the importance of text and illustration in picture book varies, usually in accordance with the age group the book is aimed for.
When I create a new picture book, I usually start with writing the first draft of the manuscript. It means I start with the words. Of course during the writing I always keep in mind the relationship that will need to take place and try to leave open span for a variety of interesting pictures. I would say that generaly it is best to convey the action and dialogue of the story by text and let the illustration describe the scene, characters and the said action, or even add little understory of its own.
Here are some of my tips what to include in the text to keep the reader involved with the book:
Include scenes with action. The story should be vibrant, create a feel of unexpected and keep the reader wondering what comes next. Good action sequence will escalate the tension the story creates and leave the most graduation of it to the very end.
Introducing new characters to the story will also attract attention of the reader, but I would advice to keep in mind, that most of picture books for little kids don´t have an abundance of characters in it. Also, make sure that the character is neccessary for the story to be told. Picture books are short and there is no space for diverting the pace of the story without the risk of loosing the child´s attention and fascination with your story.
Other tool of keeping the reader in the story is moving your characters to different settings. Is it really neccessary to tell all your story inside the house? Or even in one room? Moving to different setting will also give the illustrator more space to work in and I bet he or she will love you for it.
And the last, but not least, is to change emotional load and intensity of the scene.
A good picture book manuscript usually has a combination of things mentioned above. Keep in mind, that the picture book is likely to be read many times by parents as well as children, so by making the story interesting and by adding a nice touch of humour, you can make sure that the book is not likely to become the parents worst nightmare after a couple of readings.