Both my kids wore shorts as toddlers - I can't' imagine not wearing them in the summer and warm weather. If they were in leggings (DD) that would have meant a lot more stains and ripped pants and still skinned knees - my kids get skinned knees through pants all the time in fall/winter when w'ere outside on the concrete anyway.
I love bike shorts under dresses/skirts for little girls as well as cotton shorts + cotton tshirts for both sexes as they're very comfortable. For DS he wore a lot of cargo shorts that were heavier canvas and DD wore a lot of sundresses with bike shorts.
Mom of 4: Boy (10), Girl (7), Boy (4), Girl (2)