When we run ours on propane we have to turn it down to 2 or less to keep from freezing stuff. On electric we set it to around 4 and a half. As Herk and all have said, it's ambient-temperature dependent. Starting at 3 or 4 as you plan makes sense to start getting the range on the beast. Put the more freeze-prone stuff like veggies on the top shelf and the stuff you want really cold on the bottom. And tell the kids it's not a revolving door, open it as few times as possible.
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator