Ask players if they know what a flamingo looks like when they are sleeping? After they've had time to respond, demonstrate that a flamingo stands on 1 foot to sleep - and they close their eyes.
Have players pair up - each having a ball.
Have them kick the ball to their partner. As soon as they retrieve their partners ball (they should try to use their foot to stop the ball), they should immediately stop and pretend to sleep - by lifting 1 leg (balancing) and closing their eyes (some people have a harder time staying balanced if they close their eyes).
Don't make them stay in this position very long - as this is sometimes a very difficult manuever for the U6 age group. Practice doing it several times. Tell them they can practice being a flamingo at home - even if they don't have a ball nearby.