I realized that despite the fact that Denton can't read yet, he can absolutely solve a substitution cipher.
The key was printed onto slips of paper, and hidden in plastic eggs around the garden. It's much easier to do than clue sequences that have to go in order, or be marked on a map, and George was able to help find/open the eggs.
Of course, I couldn't quite bring myself to encode it manually as a one-off in gimp; I have written a python script that takes the tiles as encapsulated postscript files, and fits them on the page as desired. It took a whole evening.