Easy does it! You may be excited to use smoothies as a way to get a daily serving of vegetables into your child, but most kids (adults included) won’t want to drink an avocado-ginger-kale smoothie right out of the gate.
When our big guy was two years old, I started him on a smoothie that I knew couldn’t fail: frozen mango and banana, a heaping spoonful of (plain organic) yogurt, and a drizzle of maple syrup, in a base of orange juice and water. As he grew more accustomed to having smoothies, I was able to make them more adventurous: adding different fruit combinations, veggies, nut butters, seeds, and spices (like cinnamon).
Since most kids can be picky about taste AND texture, make your first smoothies simple, familiar (no kale yet!), and invest in a good blender to prevent any lumpy/fiber-y drinks.
Easy combinations to start with: mango-banana, strawberry-banana, and blueberry-spinach (the blue will hide the green of the spinach!).