In general MIT license means, you can do whatever you want with the code. (use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software)
If you want to follow the license strictly, as long as you keep the copyright in the cocos2d-x source code, you’re good. Anything you created, you own the copyright.
Although, it would be great, if you can add cocos to credit screen or something, but it’s totally up to you.