This is probably not the right place for such a question.
But I am a developer, and this part here:
How do we make sure that the people we approach do not run off with what we pitch them?
It's not a realistic concern. Or rather, if your idea is so good that this is a realistic concern, you should learn how to develop the app yourself. But I really, really doubt that is the case. Generally, ideas are worthless if you can't execute. Presumably your idea relies at least somewhat on your knowledge about psychology, and probably other knowledge/skills you have that a dev won't have.