I was given a Japanese Red Maple and a Hawthorn seedling. Both are about 6 inches tall and outside in separate pots with their native soil. I would like to develop them into bonsai. Should I let them grow bigger? Should I bring them inside in the winter? I live in Northern NY with extreme cold. I have had bonsai in the past but never had a seedling and want to do the right thing. Can you give me some suggestions, PLEASE. Thank you.

You should get some advice from Bill Valvanis, International Bonsai Arboretum, in Rochester, NY. He can give you particulars about what he does for winter and seedlings.
As far as developing them into bonsai - put them in a pretty large container and wait five years, at least.