1) Take a deck of cards.
2) Fan them out in your hand face down.
3) Have a kid pick one, any one.
4) Split the deck, make sure you remember the card on the top half (the one in your left hand after you split) DON'T stare at it, just casually glance it out of the corner of your eye, you only need to see it for a split second for your brain to register it.
5) Have the kid put his or her card face down on the bottom half.
6) Replace the top half face down, the card you memorized is now on top of the kid's card.
7) 'Pretend' to shuffle. You know, that method that people that can't shuffle for real do, where they slide them around. Only keep swapping from both outsides of the deck, don't go more than 10 cards in on either side. The middle cards stay the same.
8) Say you'll find her card. Start sorting through the cards, make it all dramatic like, once you get to your card, you'll just have to go one back to have hers. Or if you start sorting from the top of the deck, it'll be the card AFTER yours.
Very simple and 'duh' trick, but young children usually won't get it until you explain it to them.