I made up a quadruple batch and cooled them in three 9X13 pans (lined with greased wax paper.) When I put them in the pan, I pushed it down with my hand (coated in cooking spray) to make them more dense and sturdy.
Then I cut them into very small squares. We ended up with a lot extra, but the kids should have built bigger walls so it would have been a good amount.
For mortar we used jarred frosting.
Tertius just stuck a bunch together in a random order. But, boy, was he proud!
When we placed the girls' next to each other it made a longer wall. This led to a good review of how the Great Wall was composed in places of previously built walls linked together and repaired
It was a fun activity. Even Quartus got in on the action (the best part-eating it!) when Tertius dropped a big chunk on the floor.