There is a really simple but effective trick I do with the children that foxes most people at first - very simple to do, and easy to teach too a child
The layout:
Get any 9 playing cards - make sure one of them is a 9 (any suit, lets say diamonds)
Look at the pattern of the 9; 4 diamonds on the left, four on the right and one in the middle
Lay the actual playing cards out on the table the same way as the 9 diamonds - to make it easy to follow put the 9 in the middle (like the diamond on its own in the middle of the card), then any four cards in a vertical row on the left and the other four in a vertical row on the right forming the same pattern as the 9 diamonds in the middle of the 9 of diamonds card:
eg
A------5
8------J
---9---
Q------7
6------K
The Trick:
Get someone (the mark) to point to a card while the child isn't looking/is out of the room etc, Lets say they choose the Ace in the example above
Call the child back in.
The key to the trick is to point to a few cards that are not the chosen one asking if this is the card. Its important to physically touch the card
At some point carefully point to the 9 in the centre.
When you do this touch the diamond on the surface of the card that corresponds to the actual card chosen by the mark.
In this example, because they picked the Ace you touch the diamond in the top left of the pattern.
The mark will think you are just identifying the 9 of diamonds (and hear the child say no) while you've just communicated that the actual card is the Ace.
Example - staying with the Ace for now - child comes back in the room, you point to the K and ask Is it this card - child says no. Point to the 9 and ask is it this card - child says no (child now knows its the Ace). Point to the J and ask is it this card - child says no. You can then either point to the A at some time (child says yes) or just ask the child which card it is.
If the mark picks the Q next go, then you point to the diamond third down on the left and so on
If they pick the 9 itself then you have to point to the central diamond and the child simply says yes, its that card.
There are lots of variations of how to do it - we've had hours of fun with it and its simple enough for the children to pick up pretty quickly.