how to save a dying seedling
The seedling we planted is dying.
I can't tell what kind of tree you've planted, but you need to consider whether the sapling has the correct amount of water, if the soil is the kind the species needs, the amount of light is correct. Also consider things like the process you used to plant the tree, the possibility of disease or pests infecting the plant before you planted it, or soil-borne organisms that are attacking the roots. I'm giving you an article about planting a container-grown tree, which would apply generally. https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm