Hello and welcome to the forums!
If you intend to just present the data "per device", in other words, as you said, the independent variable is the type of device, then you can ignore the voltage and frequency changes for that graph / analysis (for a project at the elementary school level).
Your son should look into whether the power values are different when the voltage reading is different. On way to do this is to measure a few devices at different times of day. In the middle of the night (may not be practical for an elementary student
) voltage is highest and in the middle of the afternoon lowest (usually). If you can get several readings on several devices with enough different voltage readings, this would be interesting data to include with the project. At a minimum, your son should be able to discuss this with the judges.