I introduced Edmodo to my ICT club this afternoon and they enjoyed it. Basically, it's an online learning environment that is private to you and your groups. You give your children a code when they sign up which enrols them on your course automatically. The children found it difficult to find a username that hadn't already been chosen, but after some perseverance they all got on, children from year 3 to year 6.
Their first activity was to upload a file as a response to an assignment. Some younger children had problems doing this, but the other children got in to it instantly and I gave them feedback. They also took a vote on whether Edmodo was a good thing and over 80% were in favour.