We just got back today. :-( Were fortune to have swimming weather every day but one! Hope the weather stays warm for you as well.
For the Volcano slide, there isn’t a height requirement. And yes, there are normal life jackets of various sizes available. At 4 we’d put our DD in a life jacket and just hang around near the bottom. Yes, one adult can walk up and another can wait below. You can see the top waiting area if you’re a bit away from the splash down area, but you can’t really see the top waiting area if you’re closer to the splash down area. We’d usually just hang back a bit to get a visual on when she was coming, then quickly step / swim over closer to the splash down area. The water force will ‘push’ a child in a life jacket towards you at the bottom. This will make more sense when there, it’s pretty easy.
At 4, I suspect your DD will love the kids play area as well. There are two slides of smaller sizes that are perfect for many kids.
We took a last morning swim this AM at pool opening, DD5 was in tears as we had to go in to leave. My kids have always loved the Poly pools.
Hope you have a great trip.