Blockchain cipher suitable for preschoolers Is there a cryptographic hash function that well suited for preschool-aged children to work out on pencil and paper, to learn concepts of block chains?
It must have the following requirements:

Able to be easily verified
Able to be easily calculated with pencil and paper
Able to link to previous result, to create blockchain

The function may be low-complexity. It does not matter if the function is easily broken.
If all you want is a simple hash function you could do something like this. 
First take your original message and apply something like ROT 13 to it. This gives a reversible cipher, of course. Then give each letter a number, say N = 1, O = 2, etc, wrapping after Z. Next, just add up all of those values. It is no longer a reversible cipher, but a hash. To get it to be fixed length, simply pad or truncate it to, say, five digits. 

Pretty bad cryptography, of course, but simple enough if the students can count and add. 

The use in a blockchain is up to you. 

This hash has the features described here: https://en.wikipedia.org/wiki/Cryptographic_hash_function, but they aren't very strong, of course. 

Fixed Length
Deterministic
Easy to Compute
Infeasible to Reverse
Change in the Message changes the Hash (not a lot, though)
Infeasible to Generate a false message with the same Hash (maybe). 

Call it the puppy-1 hash function if you like. 

And of course ROT-13, like the Caesar Cipher, has some historical interest. 



In the first part, ignore case, punctuation, spaces, etc., which isn't sufficient in general, but should be for this use.