Hi,
They are all bantams so don't take that much room! I leave them out all the time, they have only just turned a year old and where fine in the winter, as long as you have a good house for them, they will be fine. Although in the winter i did get the hair dryer to defrost some icicles in the house and gave them some warm(ish) porridge!
If you still want to keep them inside over winter, just keep them in for a month at the most. And it's June tomorrow so its very unlikely to snow!! Also they will be happier outside, it is more natural for them.
Sorry just seen you said they are 12 weeks! If they have got all their feathers, then they should be fine but when there is a risk of frost I would bring them in.