It depends, IMO, on what exactly you want to teach. A fitness certification in kickboxing will likely be a cardio class that pantomimes actual kickboxing. I have a fitness kick-box certification, and that’s exactly what it was.
I learned a heck of a lot more in four and a half years of taekwondo (to black belt) than my one-day kb certification.
Some kick-box gyms will train you in their class formula so you don’t need to be certified, just trained by them.
No matter which route you take, you should have enough knowledge to perform and teach the moves safely, to start them at the most basic level for beginners and people with injuries, and to progress them up several levels of intensity.