Different types of elastic behave differently. Try stretching it between your hands.
Is there a lot of give? You’ll want to cut a bit shorter than the waist measurement and overlap the ends by an inch or so to be sure it stays put.
Doesn’t give much? If you’re overlapping the ends to attach them you’ll want to add the amount of overlap to the waist measurement so that it doesn’t feel constrictive when it’s finished.
And the chart for big kids!
Remember, these are averages. If you’re sewing for a particular kid and can measure them or have someone else measure them, you’ll get the best results working with real numbers.
Are you sewing skirts? See this post for average skirt lengths for girls!