Here’s one that my uncle used to pull on us. It does require a confederate with access to a phone.
You get the child to select a card at random and show it to everyone. Then tell him that you know a wizard that can name the card. The child picks a card, six of clubs. You then call the wizard. He answers “Hello” You reply, “Is the wizard in please?” That is his cue to begin naming the suits with a slight pause between each. When he says “Clubs” you say “Sure, I’ll hang on” or " This is Jeff" or whatever. He then begins going through the cards “2,3,4.etc.” Upon reaching 6 you interrupt and say “Here he is” and start to hand the phone to the kid. The wiz may confirm by saying “six of clubs” to which you may reply “Oh, his name is Billy”. The wiz can then play it up any way he wants but in the end he names the card. Once you have it down you can call you partner anytime (although he may not appreciate it). We could never figure it out. It never occurred to us that the wiz never answered his own phone. Even that can be gotten around with just a little planning. I never tried it on an adult but I don’t see why it wouldn’t work.