I can't really respond to that without bringing my own daughter into this. When she was around 3, she was in a public swimming pool when she suddenly noticed the power of the water jets. "Look, Mommy! This feels GOOD!" After that, we had to have a long conversation about what is appropriate public behavior and what isn't. On the one hand, you don't want to shame your child for a very real biological reaction but, on the other hand, you can't have your kid straddling water sprouts everywhere they go.
The fact is, there are lots of nerves down there. The right kind of pressure on that area does feel good; we're partially built that way so that intercourse is enjoyable for us. It is weird seeing a young girl inadvertently discovering that certain things feel "nice"? Yes. It's icky. Does it mean that she is being sexually abused at home or has any kind of disorder? Absolutely not. We talked about this repeatedly with our pediatrician who assured us that this is normal-some female children just "discover" it far earlier than others. I have also spent quite a bit of time on the internet, perusing parenting forums and looking up parents with similar questions. It happens. A lot. Sometimes it's with hot tub jets, other times it's discovered while straddling the armrest of a couch while they're playing "cowgirl." But it happens.
Also...we have a season pass to a public swimming pool on the island in which we own a vacation home. We go several times a week. The pool has a splashpad like this. I see this kind of behavior at least once a day. Seriously, it's pretty normal.
It IS kind of weird, though, even for the parent. Since that first day we had to have that conversation with her, we've had to have a lot more. Now that she's 7 she is finally old enough to really start grasping what we're saying. Now she's totally embarassed by it. To be honest, I kind of miss the toddler "her" who was just innocently playing and didn't realize that she was doing something that others might think of as weird.