I think around age 6 is when kids start to recognize the different types of cards. For 7 and 8 year olds, they recognize cards, but will not be interested in most card magic.
As Rick said, you can do some routines with cards that have pictures on them -- Rachel Columbini has a great effect with cards that have animals printed on them -- I've used it with Pokemon cards and others have used it with cards with pictures of Disney Princesses and their Princes.
Part of the problem (and I was discussing this with someone just yesterday) is that kids shows are more of a parlor show than a close-up show. That is part of the reason for using Jumbo Decks. Any card trick you perform has to be visible to people sitting in the audience.
The card effects that I use for 5 years old and below include include Mark Wilson's Bigger Card and Rachel Colombini's
Animal Capers (trying to use an older child as the assistant). Oh yeah, and there's the trick with my sheepdog puppet picks the card that has his bone on it out of a packet of cards held by the spectator
For 6 years old and up, I'll use an Invisible Deck (sometimes Jumbo), Jumbo McCombical Deck, Dan Harlan's Hovercard and a signed card to impossible location. HOWEVER, each of those has been transformed from a simple routine into a scripted performance piece that takes 3 to 4 minutes. The invisible deck routine involves tossing out a beach ball and having the kids name part of the card - and yes, I've been doing that before Matthieu Bich did it on Fool Us
. The Hovercard involves a three minute story before the final levitation and the card to impossible location is done with one of the children acting as a magician to music. Just like everything else in magic (kids or otherwise) the secret is 10%, technical skill is 20% and presentation is 70%.