I need a little help choosing the right cipher.
I'm looking for a simple pen-and-paper cipher that is easy enough for kids to quickly decode by hand, preferably using an alphanumeric decoding table that they can quickly populate from a simple key such as a phrase or date. Ideally, the security level would be sufficient enough to prevent your average non-cryptographer from cracking it in a few hours.
We'll be using the cipher to store map coordinates, driving directions, and addresses. The intended use-case is a pre-teen child decoding turn-by-turn street names while the parent is driving them, hence the requirement for speed and simplicity of decoding. Furthermore, the child will only be given a few minutes to learn the decoding process and populate the decoder table before the drive begins.