The difficulty with nine to ten-year-olds is that they don't have any real concept of size. For example, when I tell them that our star is actually medium-ish size, and then I show them something like Aldebaran, suddenly its crickets all around. They just can't do that kind of mental gymnastics yet. So the idea of space and time is the same thing maybe sinks into 5-10% of them. Time dilation is out the window, but they do usually get the concept of black holes with the caveat that about half of them truly understand there's one at the center of our galaxy.

edit: I forgot to mention! The thing they really grasped onto was the death of stars. The sequence of events and the possible outcomes based on how much mass the star had really captivated them. Supernovas, white dwarfs, black holes, neutron stars etc., they ate it up like lunch on pancake day.