To make a smoothie be even more nutritious and a rounded meal, add oats, nuts, protein powder, or wheat germ to slow down the absorption of sugar from the fruit into your child's bloodstream.
Jeannette and Tracee (Simple Food for Busy Families) recommend that these work really well for children who have a light morning appetite, because it feels more like drinking than eating.
I can also add that it is a great way for your children to experiment: my seven-year-old can whip up a chocolate milk - carrot juice - strawberry mix in a second -- his favorite combination.