I saw this electromagnetic Thor's hammer video recently, and thought that something similar could be adapted at DL for an updated version of the Sword in the Stone:
If the hammer is placed on a metal surface (it would likely have to be in a dedicated, enclosed meet-and-greet space), Thor could occasionally challenge guests to pick it up. After the unsuccessful attempt, Thor would then easily lift the hammer himself -- though the process would probably be faster and more effective if the thumbprint scanner were to be replaced with an RFID device or something similar.
Every so often, or maybe at specified show times throughout the day, the electromagnets could be turned off, and a "worthy" child would be allowed to lift Mjolnir.