I think the enclosed part is shorter than the open part, maybe they are about equal. I don't think it is longer. When you stand at the top, you can see the daylight at the end of the tube. It' s not really that long. It is big enough for a small child to sit up on the way down. Adults and taller kids will need to lay down.
If you lay down, you start out slow, but gets quite fast near the end of the tube.
After the tube, you have a right curve, then a little straight, then a left curve then a dump into the pool. There are two waterfalls that splash you along the way.
If your child is able to go on the BWV slide, then they might be OK if they get up to the top and see that they can see daylight at the end of the tube. Once my DGS saw it and had it explained he was OK with it. But looking at the slide from the walkway, he was not.