I second Marvel Legendary - it is a combination of deck builder (like Dominion) and TCG (like Magic/Pokemon) and is semi-cooperative. Makes it fun for kids of differing levels as you can help them both in advice and in your own playing to beat the Big Bad, but you also can compete for points against each other. There are other versions like Alien/Predator and even Big Trouble in Little China, but I recommend the original and/or Civil War to start.
There is also
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truth
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Summoner Wars from PlaidHat games - combines a TCG and Heroscape!
Of course, both of these are "Expandable" as opposed to "Collectable" which I highly recommend getting for kids (as well as yourself) as the "Got to Catch 'Em All" feeling won't break your bank as you only need to buy one set of each as they are not random blind purchases.