arrange heaps of sticks and tell them to make hexagons with it....join everyone's hexagons to make a huge domical shape.....children should like it.....

One good way to grab kids interest is to get a few laptops with a sketch up model on that they can navigate and play with.

Another option would be paper bridge building contest using old newspapers (prompting ideas of recycling?) and sticky tape, while this can be seen as engineering it's structural focus is also a good tool to promote architectural thought as well.

You could also hold an open design competition to rethink something as simple as a bus stop? This sound simple but if you can get the kids thinking laterally about design then they might come up with some great ideas (older teenagers tend to turn their noses up as this as it seems simple but you have to explore diverse ideas from simple shelter to relaying information - the best scheme one of my work experience students came up with was a bus shelter rethought as a funky I-Pod dock that displayed videos based on the tunes you were listening to while waiting for a bus)

Lastly maybe an observational treasure hunt to get the kids to look at buildings (most times you make somebody look up from street level they are surprised by what buildings and details they see!). This could be made more interesting by letting them use mobile phone cameras to gather the evidence? With this you can set up a route and also promote team building, another important corner stone for working in architecture (well in theory!)

Good luck