My oldest dd took the EXPLORE at age 9 as a 4th grader. Like you, the only prep we did was having her do the sample questions from ACT. In her opinion, the actual test was much harder than the sample questions. I'm not sure if it was the set-up of taking it a a university or the time pressure or whether the actual questions were harder (which is what she seemed to feel).

Study Strategies for Accelerated Learners