For an 8-year old, we do not recommend putting your child on a weight loss regiment. While there are some children who can benefit to be at a healthier weight, we should approach it differently than from an adult.
What we would recommend is having a family approach to healthier eating. Often times, parents want to help their kids and put them on a strict regiment. However, it may benefit for both the family and the child that they do it together. Include 5 servings of fruits and vegetables in your diet, decrease the sugary beverages, increase whole grains, and increase the amount lean protein. This can often help kids feel more included and not an outcast in their weight loss.
If you really want to go through the smoothie route, make a smoothie with your child and have Parent-Child time and try one of our NutriBullet University recipes. We use these recipes for our five schools in our school-based nutrition program. I would recommend providing 1/2 the smoothie for your child and 1/2 for you to start. It can act as a supplement to a snack or a breakfast.
You can find the recipes here: https://www.nutriliving.com/search/nutribullet+university
Thank you :)