The information provided here is more detailed than I would recommend you use with 9-12 year old students. Terrestrial planets are small rocky planets which may or may not have any atmosphere. Jovian planets are very large and massive, and are primarily made of gases. Planets form hot, and lose the heat from their formation over time.
More massive planets cool more slowly than less massive planets.
Measure the temperature at the center of each potato. The smaller one should have cooled faster. This experiment may work just fine as a thought experiment. The model I have presented here is very simplified. The ways in which a planet loses its heat are also extremely important to the time it takes them to cool.