Writing a novel with another person seems really hard. I would imagine the way to do it, would actually be to create a pretty detailed outline first, so both parties truly understood where the story was going.
With the screenplay, register it with the WGA ($20 I think) and then you start sending out a single page query letter to literary agents in LA.
Children's books are technical both one the easiest and most competitive writing fields. The biggest problem is that, yea I'm guessing you'd write something way off the commercial children's path. So I think the best way for you to go is to try and either build up a web presence with the book or self publish.
self publish sounds best. and cooler because i could draw my own pics. the law up here is wack and writers can't draw their own pictures or choose their own illustrator... it is picked by the company