They’re a great source of many nutrients, particularly potassium. Magnesium and B6.
Did you ever wonder what happens as they change in colour?
Green à Yellow à Brown as the banana ripens:

• Resistant Starch content decreases
• Tastes sweeter – resistant starch turns to sugar
• Gets digested easier and quicker (causing a spike in the blood sugar)
• Some of the micronutrient levels decrease. This happens to all produce to some degree once harvested.
• Antioxidant levels increase

For babies and toddlers, really green bananas may be too hard on their tummies and can cause or make pre-existing constipation worse.
They are a great source of prebiotic.
If you want to speed up the ripening process, place it in a brown paper bag. If you want to slow it down then place it in the refrigerator. The peel will turn brown but the inner flesh will be just fine.