Children (and adults) learn much more when they work in pairs on a computer. This does not only concern programming. The reason is not only that two see and know more than one, but working together enforces (verbal) communication, which is crucial for learning and understanding.
Using Code Monster some pupils will move forward before they have even tried to write the code themselves. Encourage the pupils to think and try first. In order to encourage thinking before trying (trial and error) you as a teacher/instructor could give the pupils a new task every now and then, whereby they have to write the code (on paper?) first. Show the different solutions to the class (using a projector or a SmartBoard) and discuss the different solutions. Afterwards check the result in Code Monster.