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.
So rather than a chart based on your child's size, I figure it's easiest to measure a pair of your kid's favorite leggings then compare with unwashed Smarty Girl leggings laying flat:
The above chart is for Smarty Girl's collection of 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 a bit if washed in warm and dried in the dryer. I recommend washing in cold and laying flat to dry.
Dinosaur, airplane, shark, insect, chemistry: