Acknowledging those points along with some constructive criticism helps the students learn some of the more difficult points. We have found that group discussion following testing in which each team or individual critiques the strong and weak features of their own design is fruitful. Finally, please make a special point of reminding students that this kind of creative activity is at the heart of engineering. If they found the project enjoyable, they should consider engineering as a profession. Please be very careful while timing the students so that they don't get carried away with doing well and get a piece of their product stuck in their throat.
Cleanliness is important.
While building the IES, be sure the students have adequately cleaned their hands and that they use only clean equipment to be sure the scale is fit for eating.