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I have some kids that like encryption schemes such as the Caesar cipher and the Vigenère cipher. I would like to teach them something that's not easily breakable by todays maths and computers, but I want them to be able to use it just using pen and paper.

RSA probably isn't the way to go, since specially the prime finding part is quite intensive.

A symetric cipher like AES would also be fine, but the current AES standard requires lots of rounds of calculations to be effective.

One-time pads are nice, but not really a proper cipher.

Are there any cute modern schemes that are really cheap, but still have a reasonable amount of security?

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    I also found this thread: http://crypto.stackexchange.com/questions/844/is-there-a-secure-cryptosystem-that-can-be-performed-mentally2012-10-17

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You might get a kick out of the solitaire cypher that appeared in Neal Stephenson's novel, Cryptonomicon. The novel itself might be heavy going for kids, but the cypher is cool, since it's not only pretty robust but, even better, uses an ordinary deck of playing cards.

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    Could one also use rc4 reduced to a size 56 (deck of cards) state?2012-10-17