For me smoothie is a super easy way to add more vegetables (especially leafy greens) fiber and other nutrition from "optionals" in one's diet. However, most children will not have just any green smoothie, it must stay tasty and rather on the sweet side. Keeping the proportions in this template will yield success most of the time.
2 cups liquid (water or some nut milk)
2 cups fruit
1 cup vegetables (loosely packed)
If not adding any frozen fruit, add about 1/4 cup of ice. Add some creamy ingredient such as a banana or avocado. The ice also makes it kinda thicker and creamier, tastier. Adding healthy fat as bellow, helps to carry fat soluble vitamins and makes the smoothie more filling, so does protein.
Optional: add 1-3 Tbsp
Experiment. Obviously, don't add stuff your children don't like, or don't do it next time. Use milder tasting vegetables.
I can't do olive oil and pungent herbs such as coriander (which I personally love), they will taste even just a little bit of it. But, possibility is to blend it clean for the kids, pour their glass, then add more veggies or whatever else you adults want in it.