Lisa Barnes from Petit Appetit mentions that the best way to create smoothies for young children is to use frozen fruit instead of ice, which can be too cold for children.
Smoothies with Oatmeal
To make a smoothie be even more nutritious and a rounded meal, add oats, nuts, protein powder, or wheat germ to slow down the absorption of sugar from the fruit into your child's bloodstream.
More on Smoothies
How you can involve your child, how you can make smoothies more substantial, how you can add variety, how you can make smoothies more appealing.