I was reading an article with tips on WDW trips, not expecting to learn anything new, but this one surprised me.

If your child’s too short to ride something—height requirements are on the guide map, and there are rulers before every queue entrance—ask the nearest cast member to give you a voucher that enables them to cut the line when they grow tall enough. For Splash Mountain, they’re dubbed a “Future Splash Mountaineer,” and on Space Mountain, they’re handed a “Mousetronaut Certificate.” Many other major rides have their own versions.