I tried writing a television treatment. Failed miserably. But that is what production studios - not networks - demand. And even if you get accepted, you have to be prepared to work with the in-house writers. The can be a good thing, in that you have a crew to help flesh out ideas, scrap the stupid ones, and meet deadlines in case someone else gets sick. Also, there is the possibility they can sit on the treatment for a decade, and then decide to release it long after you may have lost interest.
And unless you somehow get the producers to agree on writing a contract that says "Do not sell to Nickelodeon!", they'll probably try to go to Nickelodeon.