Thank you for the following up post! Sorry to miss your post yesterday. I’m looking forward to see more details of these mystery saplings in your new post.
Among another possible ID is something in the Nyssa genus. I hesitate to include this in since you had mentioned these are vigorous in growth, according to sources, Nyssa’s growth is slow to medium growth rate.

If you could give other details i.e. the side view of one of larger size specimen to show its twigs or branches position from the main trunk. Also a closeup of its unopened leaf buds. These might help us key out its ID for your. Thank you & best wishes!
On the side note, to add additional pictures, you need to create a new post.

I submitted another picture in new posting. Didn’t see how to delete/add.

It’s hard to tell at this sapling stage since there are few plants with similar looking leaves i.e. species of magnolia , American persimmon, etc… Persimmon is growth rate is more vigorous than magnolia. You had mentioned: “their stem will actually harden up.” sounds like these are woody type plant (tree, shrub) and not a herbaceous plant. Maybe you could let one grows few more months and resubmit with new photos. Thank you & best wishes!