You should do everything you can to get your under 40" child on a ride where they have to be over 40". Put in gel inserts, wear heels, have them stretch, let their hair grow. The rules make no sense-your child is special. You can't handle saying no to your kid-what a mean parent.
After something happens to your kid, you can then sue WDW for not properly enforcing the height requirement. They will then be big enough next year, and you will have lots of money courtesy of WDW-assuming your child is alive and not disabled.