I agree the answer by Harris S. I have been working at a Y as a trainer for 11 years now and the exposer you get to pass on healthy lifestyles to kids is immeasurable. I also realized that without making an impact on the parents what you teach is less likely to stick. One of the big events that Y in my town, Columbus Ohio at least, does is stag an annual Healthy Kids Day in late April and early May.
The only thing I can add to Ariadne’s writing is for you to look into the YMCA. They are kid, family and community oriented and I believe that would be a great place for you to start a fitness career, especially if you want to work more with kids. There are many other places and organizations where you can do that, but the Y has developed many great programs targeting the younger population.
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.