I know this must be an elementary question, but we are an elementary
group (middle school). We have an old donated door that we wish to use
as a prop. It will stand alone (not attached to walls) and will not need to
be opened. We need an easy way to roll it on and off stage (It's very
heavy), and we need to feel that it is safe enough for 10 years olds to be
moving about without the worry of it coming crashing down. Can
construction advice?

Fix it to a truck, fitted with castors. Stage [French] brace the door to the top of the truck, counterweight the braces, using sand bags or other stage weights. You can dress the gap between the truck & the stage with a wooden plynth or use black tat masking / burlap sacking.

Don?t use cheap castors, that only have a steel bolt for the wheel to rotate on, as these will make a lot of noise, only use the ones that have bearings fitted to the wheels.

You can make up a push pull ?T? brace, with a keystone gusset, over the cross bar butt joint. This is hinged to the underside of the plinth, on the off stage side, that will rest on the stage & act as a brake. Which will also give more control moving it on or off.