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The sticky substance is likely Honeydew. Honeydew is the residue from pests; your tree is likely infested with Aphids.

This article will help you.
https://www.gardeningknowhow.com/plant-problems/pests/insects/aphids-tree-disease.htm

Does this barrel allow excess water to drain fully? These trees are usually ok with being in container and root bound for quite awhile. I'm inclined to believe that it is more likely a fungal infection. You can try treatment of dolomitic lime and wettable sulfur, but you may not notice a full recovery until next year.

If you still suspect it being rootbound, then you can pull it out of container, shake off excess soil and trim off about 1/3 of the roots, mostly the larger roots, leaving some larger roots, but mostly thin roots to take back over. Repot into fresh soil, and make sure that there is adequate drainage. This will give the tree a fresh start, as if it were put into a larger container.

I would first start with a soil test. Your County Extension Office can help you with this. A soil test will take out the guess work and the nutrition needs of your tree.

Is your soil well draining? Adding organic soil amendments can help.

These links will help you.

https://www.gardeningknowhow.com/extension-search
https://www.gardeningknowhow.com/ornamental/trees/maple/care-of-red-maple-trees.htm
https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm

What you have are "cuttings" not "seedlings". This article may help:
https://www.hunker.com/13428795/how-to-start-a-new-maple-tree-from-a-branch-cutting

If you search this term: maple propagation from cuttings, you will find more articles and YouTube videos on the subject

I don't think you are doing anything wrong. If you have Japanese maples, I know this is a common occurance.
Provide good soil fertility and water management and trust the natural processes.

https://marylandgrows.umd.edu/2018/02/28/qa-why-didnt-japanese-maples-lose-their-leaves-last-fall/

The most important thing with trees is to make sure they are watered, as needed, up until the ground freezes. Winter winds strip moisture from bare branches. A tree that is water stressed going into winter is a problem. The rest is up to Mother Nature. If you only recently planted your trees, mulch the ground over the rootball with 4 inches of straw, chopped leaves or wood mulch. Just don't let the mulch touch the bark. Leave a 2-3 inch gap. The other issue for you is how to protect the saplings from hungry wildlife. Mice and rabbits will nibble on young bark to say nothing of deer. Constructing a cage with chicken wire or hardware cloth and stakes or PVC pipe is fairly simple. Just make it tall enough to keep deer from eating your central leader. Here is an article with more info. https://www.extension.purdue.edu/extmedia/fnr/fnr-484-w.pdf
Check with your state Extension office for advice specific to your region. Search online for [your state] Extension Service.