Have you tried any of the geoinquiries? They do draw my 6th-grade students into the concept via the interface of the map and the data. The "Dust Bowl" Geoinquiry brings together data on precipitation, agriculture, population and geography.
In class, students were learning about erosion and weathering. I decided to have the students explore this Geoinquiry. I gave them the handout with guiding questions. The students soon abandoned the handout and explored on their own. I walk behind the students as they work, and I listen to their comments and questions and observe them. The precipitation graph was astounding to them. They were surprised at how small the populations were. A student used the measure tool to find out the area of the "dust bowl" in the 1930s.
Back in the classroom, I introduced the systems thinking tool: stocks and flows. I had the great fortune of knowing the late Barry Richmond, who was a mentor to me.
Here is is a snippet of the class:
Students, think of a stock as a bathtub which can be filled or drained. "Population" is a stock. We represent stocks as a rectangle. Now what causes population to increase? One response was, "Something like the gold rush which attracts people." I clarified that people migrating would cause an increase in population. I asked for other causes. Another response was that people go where they can find work. Again, I clarified that this would be migration. Someone else brought up people coming from other countries. I said this would be immigration. Finally, someone said reproducing. Okay, then I introduced the symbols, "inflows", which are double-lined arrows that touch the rectangle which represents the stock of population. In the same way, "outflows", pipes leading away from the stock make the population go down. Students shared that people who move away will cause the population to go down. I introduced the term, "emigration". Students came up with "dying" as another outflow. Now, what has to happen to change the population. The students quickly responded, "The inflows and outflows would have to be different.
This introduction helped students to think about the system and to clarify their thinking. They realized that environmental factors, such as drought, caused people to leave. Those factors would be symbolized separately, or a different systems thinking tool would be used.
This Geoinquiry on the"Dust Bowl" provided a natural entree to stocks and flows. This reflects what NGSS mandates.
If this interests you, see flex change.org