Answer: After you click the Register button you will see add a swimmer; this means add your swimmer to the lesson you just selected. After you enter the 1st child's information you can click the "Add Another Swimmer" button to enter another child to the same exact lesson.
If you are trying to schedule your swimmer in another lesson or session you may do this a little later in the registration process. After you are completely done adding swimmers to a particular lesson, fill in the billing contact information; then you will see the "Register for Another Lesson" button. This will take you through the process again of selecting a new lesson and adding swimmers to it.