I am currently having a lot of fun playing Harry Potter Lego with my 6 year old daughter(Xbox). The level of quality of the Lego games is great. If Lego Minifigures Online has the same value it could be successful.
I have to say that familiarity with the Potter franchise (read the books, seen the films) adds a lot to playing through the Lego games, with all the funny little twists to key scenes in the story. So not sure if I would enjoy playing the Online game as much for myself, but it could still mean some good times together with my 6 year old.