I overwinter my citrus in a basement here in zone 7 .
Its has a upper and a lower warmer section. The plants stay in the upper one, next to a very small window which lets some light in, they stay there if the temps outside are not low enough to start to freeze inside.. When its colder and the walls , glass and plant pots start having ice crystals ,I lower them in the lower section and close a wooden door which traps the heat in. This past winter they stayed there in total darkness for about half a month, maybe more, but it was not an block of time.
I had small cumquat seedlings overwinter fine in those conditions, thought I think sor seedlings they should have their growth been hardened.
I think under your trees they will get light, just not direct light. Thats different from no light conditions of overwintering. U should follow the temp swings that happen and depending of the variety of the seedlings ,take them in a place without heat, thought above freezing , for the coolest of weather.