My oldest is 7 and takes an engineering elective at the co-op we attend. In her class they discussed building design and bridges. Then, they built them with gumdrops and toothpicks.
My youngest was enthralled with her creations. He loved it. And he wanted to try it! It's a great fine motor activity!
You'll need: gumdrops (or Dots candy), toothpicks, and a hard surface (like the table)
Start by helping the build the base. Littles love to build but it can take a while for them to figure out that the base needs to be wide enough/strong enough to support what is going on top. So, to avoid frustration, instead use this as a teachable moment. Test it with them. Let it fall...and LAUGH about it. Failure is learning too!
Then build. Ask them to create a story about what they are building. Get out little figurines and play in the creation! Build, take it down, and build again! Save a few for munching!