So, the seedling image seems to indicate that this is a woody plant, with a new shoot from a year old (short) stem. So, it's a better shot that this could be a Winged/Shining Sumac.
Meanwhile, your older picture of the plant with brilliant fall color is likely a Smooth Sumac (Rhus glabra). I don't see any of the characteristic "wings" on the rachis (midrib) of the pinnate compound foliage, nor do I see any fuzz/pubescence on the younger branches that would be indicative of the other very similar species Rhus typhina - Staghorn Sumac.
Rhus copallina is native over a good bit of Kentucky. I have worked around and planted it in Louisville parks over the past few decades, and I have an established stand of this species and the more compact selection 'Prairie Flame'. Tough as turkey toenails, vigorous to a fault, very pleasant in texture and form, and - as you've noted - a ball of fall color fire.
