It is said "You eat with your eyes," and lets face it, most things we drink are not green. It seems weird. Even worse (in my opinion) are the green smoothies mixed with berries, which take on a brownish hue. I don't want to look at anything I will be drinking that's brown unless it is chocolate!
NUMBER #1 TIP:
Use an opaque cup or one that conceals the color of the smoothie. The first time I gave my son a green smoothie, I gave it to him in a blue tinted cup (he could not tell what color the contents were!). It totally camouflaged the color and he drink it without hesitation.
I use these stainless steel cups right now (affiliate link).
Number #2 tip:
Start with the fruit flavors they like. My kids LOVE berries, so I started with berries. At first the green smoothie was "heavy on the fruit," less on the green...but there was green! As they got more and more accustomed to drinking the smoothies, I slowly adjusted the ratio of fruit to greens.
As I mentioned these tips work well for resistant adults too!
So, go on try a tasty green smoothie and feel the benefits!