In other words, it is easy to drink too much smoothie because drinking is far easier and faster than preparing and eating raw fruits. Our hormonal systems are not geared to the speed of eating/drinking that our technological capabilities deliver. With processed foods it is easy to overrun actual caloric needs before your hormonal systems realize it has happened, our satiety mechanisms are fooled, and before you know it, you're way over your reasonable caloric budget.
If you engage your children in preparing, then eating raw fruit as-is, and reserve smoothies as a dessert-treat, then that establishes a timeframe their hormonal systems can easily accommodate. Even the youngest with minimal manual dexterity can "help" wash and/or dry fruits, and be an assistant by picking up fruits from the colander and placing on a cutting board for you.
It takes far more time, but personally that is my choice (YMMV), since I advocate a slow parenting movement similar to the slow food movement, because when you direct time and attention to a child, they flourish.
To the people here who have commented that a life without artificial sweeteners is unimaginable to them I am at a loss for words. I do not know what to say to you. I'm not saying that I have an iron will or anything–once you go without for a while you lose the taste for it, it really is that simple I think.Surely it can't be as difficult to kick as say beer or cigarettes?