I have never tried this but my first inclination is to put it outside ASAP so it can develop a root system or at least bury the pot in soil. Keep in mind that I don't have much luck with bringing plants indoors over the winter mostly because the air is so dry. I have managed to keep a lemon verbena bush indoors over winter thru its dormant period in a north window with little water. Long ago a gardener told me that certain plants need a dormant period. Apples and crab apples have been growing in northern soil for hundreds of years. I think time in a south window might cook it. Be careful about how close to the glass you put it and how frequently you turn in.

You might check to see if your state has an apple growers association who might answer your question.

My advice would also be to plant it as soon as possible so it can develop a good root system before winter. In nature, seedlings survive the winter if they have good roots. I would not take it indoors. I have just finished planting a couple of northern catalpa seedlings. Once planted, make sure to water well so the roots don't freeze dry. That would kill it faster than anything.

Thanks for the suggestions. I hope I have two seedlings to experiment with.

We are particularly worried about letting the seedling stay outside as it has lost a few leaves recently, possibly to little critters like rabbits. (Our baby hollyhocks suffered the most damage.) A forum patron suggested to use inverted plastic drink bottles to shield the plant.

To combat the dryness indoors we will water the seedling along with other in-house plants - possibly just stake one of those water globes that have been effective with other in-house plants we have. Plus we will raise the humidity to a comfortable level to ourselves anyway.

Some online sources claimed since the plant is still young, dormancy is not really required. Another source claimed apples could be an evergreen if there is no cold winter.

If we bring the seedling in, we will re-pot it into a 3-gallon container with indoor soils. It will certainly hold enough moisture for the roots.

You might get some more opinions on the Trees Forum.

I wish you & your seedling the best;
I love crabapples.

Search the forum for container mix. I would not overpot as it messes with drainage. Clear plastic around a plant (rabbit protection) could result in a cooked plant without plenty of air holes. If you want to try one inside, that's fine, but for the other I would, as mentioned above, plant outside or at the very least sink the pot in the ground. Plants tend to do better when left on their own in winter rather than depending on our over/under watering, too hot, too dry, etc. And FWIW, crabapple, like apple seedlings, rarely if ever resemble the parent.

We are trying the plastic bottle protection on the hollyhocks. Each bottle has 20-30 ~5mm holes. If the hollyhocks do well, we will shield the crabapple. Or we will just bring it late September.

We have indeed noticed that the seedling has quite different leaves compared to its parent. Since the parent has a ton of 1" messy fruit, we will be glad to take a small-fruit offspring. :)