Our local library offered "gingerbread" kits - just sign up. The gingerbread is really graham cracker but with the directions it works fine. I ordered three kits: for me, and two granddaughters.
The mortar is royal icing: eggwhites, lemon and confectionary sugar. Fortunately keeping it in a sealed container meant I only had to make one batch for the three of us. It hardens in the air.
I did mine first to get a hang of what was required then invited granddaughters to come over one at a time. #2 came first on Wednesday afternoon and #1 on Thursday afternoon.
Here are our creations.