Making a smoothie together is always a way to expose and explore !!!
As your little one gets their hands on the foods they are adding to the smoothie they are getting use to the feel and smell of new foods as they drop them into the blender.
A good chef always takes a little taste of the finished project. Using a small ice cream tasting spoon or a measuring teaspoon is a good utensil for trying very small tastes.
A fun way to expand the food your child chooses to put in the smoothie, is to add a surprise ingredient to see if the other family members can guess what that ingredient is. Children enjoy pushing buttons and under supervision button pushing can be the motivation to gain their participation in the kitchen.
Remember some of the new foods to be added can be as small as a tiny teaspoon which would not impact on familiar tastes. A small piece of spinach in a fruit dominated smoothie would not be as noticeable as a cup of spinach. Small steps forward with new foods is the best approach.
Enjoy each moment in the kitchen with your child.