If the park official closing time is 10pm, Dreams will start at 10pm. They do it this way because for Dreams to have its full effect, it has to be full dark, and come late April/May/June, its 10pm or later before the sun has fully set.
Last June, for the first time ever on our trips to DLP, we took breaks and went back to our hotel for a couple of hours in the late afternoon, so we could be out again for evening and see Dreams at park closing time (it was 11pm in mid-June last year).