How about using rice Krispie treats? You make them by melting marshmallows and a little bit of fat (butter or trex) and mixing that into rice krispies. You can make it really firm and it's easy to carve shapes out of. You then just cover it as if it were cake. It's what is used for most elaborate 'cake' models these days. She could do a cake base and make a dome (in pyrex bowl?) out of these.
Your DD could use a square cake tin for the centre piece. Then baked bean tins or similar for the side bits - cut the round cakes in half down the middle and stick them to the sides of the square one for the right kind of shape?
Then for the decoration, walnut whips might be the right shape for minarets.