I am in a similar situation as you! I created a 5th grade STEM program (called STEAM) for the last two years. In addition, I taught 5th grade ELA and 5th grade science (which I've taught for over 20 years). This fall I have been assigned to create a curriculum to push in to K-2 classrooms to introduce STEAM and I will teach 3rd, 4th, and 5th grade STEAM in my classroom. The second grade teachers helped advise me to tone down my expectations for their students, so I'm sure the activities for 1st and 2nd will have to be broken down even more.
I'm going to start the first and second graders building a newspaper tower to get them in the "engineering mode". I plan to use the NGSS science and engineering standards to help develop goals for the year. Goals will include drawing designs, testing, noting patterns, rebuilding, keeping a budget, collaborating, recording observations, etc. I'm still figuring out how to break these goals down into mini-lessons.
It should be fun!