No spoilers, but this show gets frickin awesome, and I do hope you continue to watch it. And as you said, it's a kid show that adults like. My dad and I really love it and rewatch it often.
This is also a good time to get into it since the live-action Netflix show is in production (the original creators were involved, but backed out), and Paramount created an "Avatar Studios" production company where the original creators are working on movies and new shows. They recently teased a frame from the first film coming out. There's also a sequel series called Legend of Korra that is very good as well, and is also on Paramount+.
It's up to you if you want to watch the Shyamalan movie if you end up finishing the show. I suggest you don't, cause it's really bad, and I'm not sure if I've totally forgiven him, even after Split and Glass lol.
