Dressing little babies when it's cold. Their regular stuff, warm hat. If it's really cold, put them in the front carrier and stick a blanket over it, and/or zip it into your coat. As long as they can breathe it's good.
Lots of layers is the key!
I bought a fleecy car seat cover and a puffer jacket for the front carrier as Carol suggested - need to buy more layer though - he’s growing like crazy!