Coding is about learning a new language. Some people will take to it quickly while others my struggle with the task.
It is also about training your brain to think in a different way. The sites listed in grade levels are only suggestions. If you discover a challenge that you are working in is not "just right", either too easy or way too hard, don't worry; just try a different set of lessons. There are many to choose from.
All of these sites can be used by anyone who wants the challenge of coding in December. Have fun with what is available and of course take on the 2 hour challenge.