See Children's ideas in science - they argue that one should focus on energy, not voltage
How do you combine batteries to make more voltage?
Use a voltage sensor to test different combinations of batteries.

Measure voltage of different batteries (AA, C, D...) Include some that are a bit "run down".
Measure the voltage of batteries in series. See if the combination is the sum of the individual batteries. (the math includes decimals)
??Should this include applying greater voltage to a light and noticing that it glows more brightly (more energy delivered)? It gets confusing fast, if you get into voltage, current, and energy.

Voltage sensor, batteries, paper clamps, wires