What resources would you recommend for K-5 Computer Systems Concept of the CSTA K-12 Standards? In Maryland, we hope to be funded by a grant that will allow us to write a K-5 computer science curriculum to address the Computer Science Teachers Association (CSTA) K-12 Computer Science standards.  This board and the web have useful information for middle school and high school computer science, but more best practices are needed in the elementary school arena. 

We are finding sufficient resources regarding the Programming and Algorithms concept, but there are fewer resources for the Computer Systems concept. If you use robotics or microcontrollers (Makey/Makey, Arduino, Micro:bit), share your evaluation of whether these are developmentally appropriate at the grade you used them, and how you aligned your instruction to the Computer Systems concept. If you used other technologies to make the concept more concrete, what technologies did you use?
I haven't seen a lot of resources in the US for that age group, though England seems to publish a great deal of high quality KS2 (Key Stage 2, ages 7-11) material.  What's particularly nice is the focus on Computer Science concepts and computational thinking over simple programming exercise, even at this young age.  Take a look at these BBC Resources from the "Bitesize" Series, which is a series of instructional videos appropriate to those ages).

There are also collected resources here, which includes such regulars as Scratch, but also points to some lesser-known resources, such as Code Club, which has projects appropriate for the age in various languages, and Kids Ruby, which teaches kids to program in the Ruby language with guided tutorials.