The Engineering Design Process is central to engineering. I think it is important to stress and guide the process.
I found that making the comparison to cooking was a good connection for the process. A new recipe for cookies is good story line.
Define: "Create a new kind of cookie?
Research: think about your favorite cookies, talk to people, cookbooks, on-line research. Look at ingrediants and details of portions and cooking temperatures.
Brainstorm: What kind of cookie do we want to create? Ingredients, tools, equipment needed? Based on our research, what proportions, temperature, size of individual cookies.
Build: Develop a recipe
Test: Make the cookies
Prototype: Your first cookie results
Communicate: Taste the results! Too soft, hard, flat, burnt, raw? What was good, what needed improvement? Share that info.
Redesign: Modify and refine your recipe which leads back to Define the Problem. Why didn't our cookies rise?
Criteria: Taste, texture, size, perhaps cost
Constraints: Budget, size of oven, allergies
A lot of time kids want to jump into building without much thought or planning. This often leads to a lot of wasted time and material wasted and sometimes frustration leading to a "giving up" attitude. When teaching in a classroom I tried to develop guided hands-on activities which helped develop some basic understanding principles like center of gravity, friction, air resistance, especially if it would be cumbersome or difficult to research the topic.
Graphic from catholicweb.com and I have found it on several sites.