I think that, particularly due to the grade level, I would use the numeric indicator. Although the examples are similar to a thermometer and clock (as per Unit 6 of Guidelines and Standards for Tactile Graphics), they are not in the list on page 6-2.
I think that for the volume diagram, you should simplify to a straight on view, and explain in a transcriber's note. The same note could indicate that the measurements are given in Litres (L). Move the numbers and ticks outside (to the left) of the diagram, as is done for a thermometer. For the weigh scale, it should again be simplified with a description stating that there is a watermelon and _[u]?[/u]_ bananas on the scale, but that only the dial is shown in braille. Again, you could include in the TN that the measurements are shown in kilograms (kg). The tactile graphic could then be shown similar to a clock with the ticks straddling the perimeter and the numbers (including the numeric indicator) moved outside the circular dial.
Hope this helps decide how to handle these at a lower grade level. Keep it simple!