The foil acts as a bit of an insulator, preventing energy (heat) from entering the glass and melting the cubes. The cube inside the foiled glass lasted a couple of minutes longer than the regular glass. Our next step is to cover the top of the glass with foil. Does that make a difference? We'll also place a cube in an insulated travel mug with and without a lid.
For this experiment I recommend asking your child which glasses/containers to use. Ask him/her to predict outcomes and then discuss/ask why the prediction came true or didn't come true. A simple experiment such as this allows kids to see science in action, but also to make predictions and use their critical thinking skills.