You can’t go wrong with purchasing it, but if you arrive right when SeaWorld opens, most attractions are practically walk-ons for the first couple of hours. We managed to ride nearly every coaster twice during that time and enjoyed the shows in the afternoon. You can always gauge the crowds when you arrive, and if it’s busy, just buy the pass at the park that day.
I think all the coasters are pretty great, but only you can judge what your kids can handle. None of them feel overly intense to me; the main difference is the ride position. On Manta, you're lying down, and on Pipeline, you're standing up, so it's not the usual riding position. My youngest, who was 11, only skipped Manta because he wasn’t a fan of the lying-down position, but he enjoyed all the other coasters despite being a cautious rider. There’s some good POV footage for most, if not all, of the rides on YouTube.