Like Betty said, was this their first time building? Fine motor skills need to be well developed to handle such a build with small materials. Could you have used stick pretzels which are thicker and easier to control? Also, what was the engagement part of the lesson? Did you read a story to them where the characters had to solve a problem by building a bridge? Maybe a story like The Three Billy Goats Gruff would be a great motivation for them to get the goats safely across the water by building them a new bridge and then have some plastic toy goats to see if they could stand on the bridge for a few minutes.
I don't teach kindergarten but I have gone in and done a lesson or two for the K-teachers. I based one of the lessons on The Bird Beak Buffet. http://projectwild.org/growingupwild/Bird_Beak_Buffet.pdf
I got items from the dollar store like salad tongs, clothespins, regular spoons, chip clips, and I had an aluminum pan for each group and a bag of chex mix. They tried the different "tools/beaks" to see what kind of food they could grab with their beaks. I gave them each a little cup to use as their bird belly. At the end they could eat their bird food. We discussed how each beak allowed each bird to eat certain foods. As they told me what they knew, I would ask them, "What else do you know? or What does that remind you of." I try to do a lot of open ended questions with the lower grades so I can get an idea of what they think and so they do not think I am only looking for one right answer. So when you ask, what else does that remind you of~ you may get that student who says that the salad tongs move like a pair of scissors or a big machine that they have seen. (I edited the list of supplies that I used to ensure safety.)
Gisselle, thank you for sharing your concerns. Could it be that you were a little frustrated that it didn't go the way you hoped it would? As you do more of these type activities, you will get more comfortable. Keep in mind that there will be a lot of noise when they are learning something new. If a student stops participating, don't give up hope, they may just need time to process what is happening around them. They will watch others and eventually join back in with the group. I applaud your efforts and hope other K teachers are jumping in and doing science expiriments and STEM challenges with their students.