For someone more daunted by physics diagrams than grade 3's hypothetical student, how about:
The air provides two things: lift to hold the airplane up, and drag to slow the airplane down. Coax the student to deduce that we want to maximize the first and minimize the second. Remind the student that lift increases with speed and angle of attack, and that drag increases with speed and with the area presented to the air.
Take a model airplane in your hand. In an extreme nose up attitude (exaggerate the attitude for slow flight) lift is high, but drag is very high. Remind them they had to increase the power to maintain slow flight. In an extreme nose down attitude (point the nose straight down -- even drop it on the floor if you have a durable model), you're not getting much in the way of range. get the student to agree that the best attitude is somewhere in between those extremes.
Have the student consider just the outside shape of the airplane, moving through the air. Does the angle that provides the best compromise between lift and drag depend on how much the airplane weighs, or just the outside shape? Once you get them to agree that there is one best angle, then just draw the airplane, the angle of descent and the eventual landing spot.
If the aircraft is heavy, it will need to go faster, and if it is lighter it will go slower, but either way it will follow the same glide path to the same spot.