I think it's not a good idea for a K2 kid to join both Cambridge course and phonics class.
"Cambridge" is totally exam-oriented. It means the course is preparing the students for the Cambridge Exam. As the starter course is for P.1-2 students, it would be a bit too early for K2 kids to learn. Even though they learn, they may not earn as many "shields" as P.1-2 students.
In contrast with the Cambridge Exam, phonics is a different approach. It can develope kids' phonemic awareness. Kids can relate sounds to words easily. I think this is a basic and critical skill for young learners to learn English.
To summarize, I would suggest phonics for K2 kids to learn.
Your kid may join the Cambridge course when he/she is in P.1.