Hi there;
I do hope what I write here will be of some help.If you want more specific brainstorming please hit up and I will endeavour to do better.
1. First of all set your characters on a page or two. get to know your characters. Write brief profiles of who they are what they do and their personality types. I some times down load a photograph or a picture to help me with this. The better you know your characters the easier it is to convey them to your reader.

2. Who, what, when and where: Why does the child go missing? Does the child's disappearance have to do with the protagonist?
What effect does this event have on the protagonist?

3. Write a brief out line of the plot. This can be between 2 and 12 pages long. Use this as a guideline to drive your story. You have written here that you want to write mystery/drama/thriller. try to stick to one genre if possible. You can of course use mystery as back drop to your story. And sub plots can be made up from all three. just try to to use the on e genre.
That is the best I can do for now but if you need any more help/ advise please send me a message or reply to what I have written here.

Yours sincerely

zaccat