They develop a really deep tap root in my experience. I would put them individually into really deep pots if I was going to try and winter them in a green house out indoors.
Best bet is to split your group half in 12 to 18 inch deep pots and half in the ground. They are understory trees here and mine have done well even as tiny plants in winter as long as they are dormant before frost and also providing that you mulch them deeply as they would experience from leaf litter here in Appalachia.
I have not got a great deal of experience but i have raised 3 from seed over the past 3 years. I usually plant things with compost and deep mulch and then give them lots of neglect. It seemed to work especially well for my papaws 3 out of my 4 seeds are now shoulder height.
I have read that they prefer more shade until their 3rd year and then they really hit a growth spurt and enjoy lots of sunshine. Perhaps if you plant them in a sunny location you can put a tomato cage around them with shade cloth it will cut down on winds and intense sun and can be easily removed later.
Best of luck!