Previous research indicates children reason in different ways about horizontal motion and motion in fall. At the same time, their understanding of motion down inclines appears to result from an interaction between horizontal and vertical motion understanding. However, this interaction is still poorly understood. Understanding of speed change may shed further light due to its critical role in natural object motion. This is addressed by the present two studies. Children (n = 144) aged 5-11 years predicted whether a ball, either heavy or light, would accelerate, decelerate or move at unchanging speed along a horizontal, in fall and down an incline, both in a real-object task (Study 1) and a computer-presented task (Study 2). The results suggest understanding of speed change is typically limited to the expectation that change takes place between a point of no motion and any subsequent point in motion, but not between two subsequent points. Despite improvements with age, predictions of continued speed change barely exceeded chance levels in the oldest age group. Modest effects of object mass were noted. Response time data provide further insight regarding children's predictions, highlighting similarities and differences in reasoning between motion dimensions. The overall findings are used to develop clearer ideas about the development of children's understanding of speed change as well as to advance commonsense theories of motion.