I got a Kensington Expert "Mouse" (i.e. big trackball) for Charles when he was small, because he did not have the dexterity or conceptual framework to handle an Apple multi-touch trackpad. He got the hang of it without trouble. Later he started playing with my iPod touch which he also got the hang of very quickly - the multi-touch gestures are simpler and it's easier to avoid inadvertent touches when there is a direct "touch this but not that" metaphor. He can use trackpads now but still prefers the trackball. He hasn't used a mouse as much, but it's similar in difficulty to a trackball, though I think a trackball is more friendly to small hands.