Are you still needing the LEGO Harry Potter years 5-7 achievement, O Children? I had a similar problem. If you already beat the game don't fret. I got mine as the last achievement.
Try loading your game and do not turn on any cheats. Additionally do not change your characters (i.e. leave Harry and Hermione in the clothes they are in). Then teleport into the tent from the city. If this fails start a new game then reload your previous save and retry the above.