First and foremost, my deepest condolences to the family. I can't imagine how I'd feel if that was one of my daughters.
To answer the question: You're weighed at the bottom to ensure your group falls within a minimum/maximum weight range. You're LECTURED while you're still on the ground. No, really. It's a safety spiel for which you're required to answer 'yes' at various junctures. After all, besides the rigors of the ride itself, there's the stairs. If you have a cardiac episode climbing them? They're not going to get to you very quickly. They wanted guests to be aware of every risk they could think of.
After the stair climb, you're weighed again to ensure you didn't redistribute your group for extra speed.
Smaller kids tend to get more rides than the average adult. Almost as many as they want, if they're clever. If they hang out in the standby line, they're quickly added to underweight groups.
My wife and I love Schlitterbahn and Schlitterbahn loves their guests right back. It's something you feel from top to bottom during a visit. While I haven't been swift to return to post-Ferguson Missouri, the missus and I enjoyed Schlitterbahn Galveston Island on her business trip in June, missing Massiv's opening by that much.