Little Mo vs Big Mo:
The Little Mo sleeping bags are recommended for kids ages 6 months to 24 months. They are approximately 32" in length.
The Big Mo sleeping bags are recommended for kids ages 2 years old to 4 years old. They are approximately 42" in length.
20° vs 40°:
The 20° sleeping bags are for ambient temperatures from 20°F to 60°F (-6°C to 15°C).
The 40° sleeping bags are for ambient temperatures from 40°F to 60°F (4°C to 15°C).
Sleeve-types:
The 20° sleeping bags have closed sleeves.
The 40° sleeping bags have adjustable open-and-close sleeves.
Zippers:
The 20° sleeping bags have a one-way zipper and it zips from top-to-bottom.
The 40° sleeping bags have a two-way zipper so you can unzip it from the top of bottom.