I use this trick whenever camping in colder temps. Works like a mini-furnace for kids in stroller or sled. Also works great in sleeping bag.
- Bottle – large bottles stay hot longer. A Nalgene liter bottle is plenty large.
- Heavy-weight wool socks (or towel, anything you can wrap over the bottle)
Boil water and pour into plastic bottle – make sure to seal bottle. Wrap heavy-weight wool sock over bottle.
Place bottle with child under blanket in stroller or sled, or put bottle in sleeping bag before going to bed.
I usually place the bottle between my knees to keep legs warm during cold night. The bottle is still warm when I wake up in the morning. Since the water is clean and already warm, re-heating it for breakfast (coffee/tea/hot chocolate) is quick and easy.