There shouldn't be any harmful effect and would be a good way to get your baby to eat more greens (if he/she is one who doesn't like eating vegetables).
You can refer to this article for some ideas on how to introduce green smoothies to a baby:
http://www.nourishingmeals.com/2014/10/green-smoothie-recipe-for-babies-and.html
The article suggests starting with this recipe:
1 ripe pear, cored
1 large frozen banana
2 cups water
2 large kale leaves
small handful of fresh mint leaves
juice of 1 small lemon (optional, omit if baby is sensitive to citrus fruits)
Add all ingredients to a high-powered blender and blend until ultra smooth and creamy.