Grades 4-8 is an interesting category, and I wouldn't know to what extent a successful model for online learning has already been implemented for this age group.

For a somewhat younger age group, I would suggest starfall.com as an online learning site that seems to have a number of very effective elements. One element that I found remarkable is that frequently after a "learning lesson", the lesson solicits feedback. (For example, see the end of this lesson). The feedback is extremely easy to provide -- for example, the child just picks a happy face or an unhappy face indicating whether they enjoyed the lesson. (For older kids, it might instead be a choice between a puzzled expression and an "I understand!" expression.)

In any case, I think the value of building in feedback and learning assessment mechanisms would be an important thing to consider in the planning stages.