Your children will probably soon get quite good at this. If they're struggling I even write the number to be operated on on the board in the correct columns so that they can visualise how the digits move more easily. The children then ask you questions to identify the number to which you can only answer yes or no. The children may like to cross out the numbers that they eliminate with their questions. Introducing a 'life' system helps avoid this.
Discussion comes from the many different ways to create each number.
You can change the activity by creating new rules for how the children can use the digits or restricting the number of operations to be used. Number Facts Show two number cards and get the children to give you a number fact using those two cards.