I'm not sure of the specifics of your problem, it seems to be relatively straightforward to me. I have previoulsy considered addressing the subject of measurement errors with schoolchildren in the following manner - it may be of help to you and your audience or change in subject may confuse and distract them from your topic, that's up to you to decide.
Step 1 - give all the students and length of string (considerably longer than a metre) and ask them to put two knots in the string exactly a metre apart, based on their own estimation. The smart ones will use a length they already know as a guide - most common is their own height.
Step 2 - measure the lengths achieved against a ruler and see who got closest. A leaderboard of some kind can make this more fun. This can lead into a discussion of estimation in general. Plotting a scatter plot of the measurements might also be interesting and lead into a discussion about distributions (with enough students I think I'd expect a roughly normal distribution?)
Step 3 - now change the ruler for a tape measure and get everyone to remeasure someone elses string - they will inevitably get slightly different results. Write these down and look at the differences. Perhaps now also draw out a scatter plot of the differences. The change in results should bring up all sorts of questions to prompt your discussions on metrology. Examples could be:
- Are the metre ruler and the tape measure the same length? Which of these is the most accurate measure of a metre? How can we test them?
- Why do two measures of the same thing come out different? Was there an element of judgement by eye on the part of the measurer? Did the string stretch? Did the tape measure stretch? Were the conditions in the room the same? Have the knots tightened or slipped?
- Was the level of precision used in recording the measurements the same? What is the right level of precision? Did we agree on what part of the knots we were measuring to?
- Is there a general trend in the difference results? Are they dominated by bias or random errors?
I've done steps 1 and 2 with students, along with similar experiments on time and mass. (Generally they are roughly ok at estimating a metre, largely underestimate how long a second is, and are wildly inaccurate at estimating a kilogram). Step 3 is an extension that I've planned but not yet gotten round to trying out.
I think you could also carry out similar experiments/demonstrations with your exact problem of liquids in containers as well.