Blockchain cipher suitable for preschoolers
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.