Three opaque cups or small bowls in a row on the floor or a tray. One small toy.
Show the toy. Put it under the middle cup. Lift the cup. "There it is!"
A 2-year-old will be thrilled by the reveal and want to do it themselves. A 3-year-old will close their eyes while you switch cups, then guess. A 4-year-old graduates to the actual shell-game version.
Once they're confident with 1-of-3, slide the cups around once before revealing. Use opaque socks instead of cups for tactile reveal.