@Frederick Wu thanks for taking a look and the comments.
However, I am afraid, if the kids would really love to play it.
I was actually thinking about targeting adults who are a bit above the beginner level. A few folks I tested with were adults and they enjoyed playing it. Some said that the bugs were to scary though ;-)
The illustrated examples appears all low level errors, which could be detected and assisted by WL grammar color system easily.
If I understood what you mean by "grammar color system" correctly, then even those examples I are showed in the post already undetected by it - there is no colors in front end highlighting these:


Moreover a few testers suggested I use an optional indicator to show where exactly the code is missing, --- as a hint to help the solution. I also think those cases that will trigger "grammar color system" are quite good because learners need to have a good habit and understanding how code highlighting works to catch errors during a real programing workflow.
In future, the kids are very very very smart. That means they would become boring very quickly either. If we want those kind of projects get to work, we should design it really playful. Maybe, we should ask and test with kids.
Yes, if you mean young kids, they would probably need another type of game or at least some better game-play dressing for this idea. I'd love to hear your ideas who to make this work. If you come up with anything please share.