Which Morrison Outdoors sleeping bag should I get?

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.