We had a similar experience with DD10 last year when she took the Explore. I'm not aware of any Explore specific practice tests, but any big bookstore will have grade specific standardized test prep booklets. If what you want is to give practice at tests where she might not know everything, you could just use one of those from the 7th or 8th grade levels.
We used one last year: in part because this was her first formal test of any kind (we HS), but also in part because she can panic when things don't come easily. We mostly focused on repeating (perhaps comically often) that she was doing it for the experience, that there was absolutely nothing riding on it, and that we had no expectations beyond her giving it a try. In the end she did well and didn't panic so those two things together seem to have helped for her.
