The trees need to remain outside but they also need protection from the cold. The easiest way is to just dig a hole in the garden soil and sink the pots in up to their lip. Really no need to water - rainfall and snow will take care of that. If for some reason that is not possible, the little trees can be stored somewhere overwinter where it stays cold but above freezing - garage, unheated basement, root cellar. Light is not an issue but you want a temperatures that remains right around 35-40F.

btw, it really doesn't matter what kind of Japanese maple the seedlings came from - they are not the same as the parent tree and are just generic species JM's.

And watch out for the variegated leafed plant peaking around the base of the container in the bottom photo!! That is chameleon plant. Houttunyia cordata, and it is an incredibly aggressively spreading garden thug.