From what I’ve seen, most kids eventually leave for other reasons...
Kids get bored. Once that initial learning a lot of new stuff ends and far more repetition than new material really sets in, they feel like they’ve learned enough and get bored with it.
Kids have a lot going on. They play sports. They get a little older and discover the opposite sex. They get serious about school. They want more time to hang out with their friends. The busier they get, something’s got to take a back seat.
Kids grow older and move away. I didn’t start until I was 18, but there were plenty of guys around my age who’d been there quite a while before me. I left to go away to grad school. A few left to go away for undergrad. Two left for the military. Two others went to the police academy. There were a bunch of us between 18-22 or so who trained as kids then left for that stuff. That was my former dojo.
Then there’s getting a bit older and life just gets in the way. Starting a career, family, etc. I guess that one depends on your definition of kids though.
A lot of people at my current dojo started due to their kids. They watched their kids train for a while, then started training themselves instead of sitting there and watching. Most of them stayed after their kids stopped.
Others at our dojo trained as kids, left for various reasons, then came back. Out of all the adults, there’s very few at my current dojo that don’t fit into one of the last two examples. We’re pretty small - about 20 or so adults, so it’s not the best cross-section of karate society.