First off, you're doing a great thing by getting involved and preparing your niece for the future! I would have her go through the basic courses just to get the basics concepts down (just like you said) and then you can experiment in any of the code studio playground courses, or also sign up for Scratch and she can see others games and recreate ones herself. The key right now is exposure to this "computational thinking" so that she is better prepared down the road. There is no one correct path, whenever you feel comfortable discussing lessons is a great time to start.
Also keep in mind that a lot of these courses the teachers in a classroom of 20+ are learning for the first time as well, so the courses are self directed and your niece is lucky to have someone that wants to engage one on one with her to discuss what she learned and apply it!
Let me know if you have any other questions, there are plenty of resources to help out in you and your nieces coding journey!