In the New/Edit Task screen directly under the toolbar, you'll see a list of 'After Save' options. Select the 'New Task' option from that list.

When you press the Save button, you will be brought back to the New Task screen so that you can enter your next task.

Tasks Jr. even remembers the parent task (if there was one) from the previous task to make it easy for you to quickly enter a bunch of sub-tasks.