A Contemporary Story for 5 Year Olds

I have slightly more than a germ of an idea for a book aimed at the 4-7 year old market. I can provide a framework for you to get started in terms of plot and some characters and a 'feel' for the story but what I really need is fleshing out of characters and suggestions for where the story could go to maintain the interest of the reader. Not looking for cutesy but more of something with a humourous edge. (Think a slightly more palatable version of something as irreverant as Garfield). I can provide much more detail to the winning bidder but at the moment I need to confirm that you have experience of taking a 'concept with a bit of backbone' and can use your initiative and imagination to run with it to produce something of substance. I won't expect to take a back seat during the proceedings, I will provide you with as much detail as you can take (you just have to think up the questions!). This is possibly going to be a submission for my first children's book (I don't mind admitting I'm lazy) so please only accept this if you're prepared to relinquish copyright of the material to me. I expect the final product to be around the 30 pages in length (including illustrations this realitically works out to be around 15-20 pages of text?). I don't pretend to be an expert (yet) in this field so will ideally need guidance from an experienced hand.