Smarty Girl fits true to size.
The fit is similar to Target's Cat & Jack leggings. But you want to be sure. And wrestling a toddler with a tape measure? Sounds...not fun.
Rather than a chart based on your child's size, measure a pair of her favorite leggings then compare with unwashed Smarty Girl leggings laying flat:
The above chart is for astronaut, robot, narwhal and vehicle leggings. Sloth sizes 12-18m through XS 4/5 fit slim in the waist and long in the leg; consider sizing up.
When in doubt or between sizes, buy the larger pair. Better to be a bit baggy at first and fit more snug as kids grow.
Leggings may shrink if washed in warm and machine dried. I recommend washing in cold and laying flat to dry.
Size chart for Smarty Girl airplane, shark, insect, chemistry leggings: