To work in fitness you want a nationally recognized certification either in personal training, or group fitness, or health coach. ACE is one certifying organization that is well recognized, though there are others. You will need to read and study fairly intensively to prepare for the testing. A good certification will generally have a proctored exam, and the material will be challenging.
Alternately, or in addition, you could get a degree in a fitness related field.
Once you have the basic certification you can get a specialty one in youth fitness. Again, look for one that is well recognized. The ACE site has a tool to compare various certifications…. even if you wanted to do a different cert., looking at that can be a good way to compare options. You probably wand a simple ‘youth’ specialty, though you can add to that if it seems warrented.
Currently working with kids will give you a good leg up, in that you will have connections, and experience with this population.
Once you have the training, information, and certifications you can look for internships, or other work opportunities. It can be helpful to work in the industry for a bit before starting your own business, though if your connections are strong, and the opportunities ripe, and you have some savings to cushion the transition that can be a way to go. I started part time while still working my full time job, until I was ready to transition to full time and that worked well for me.
Aside from being certified and insured and really learning the material there is no one right way to go from there. Do be open to opportunities. But generally better not to waste money on inexpensive open book online tests. Spend wisely on your training and it will serve you throughout your career.