If you want to provide your members a discount for registering multiple players you can setup a family discount in your program under additional features.
In your program go to the additional features section and click the add feature button.
To setup the discount you will need to setup a category that this feature will fall under. You will also need to give the feature a name. After that you can set the number of child that must be registered to be eligible. If the discount is available after the 1st child then set the prior registered family members to 1. You will the set the price to offer a discount be sure to include a minus (-) before the value of the discount. You can enter a description if desired. Once you have all of the information entered you will then check the boxes for default selected and active. This will make this option available for your parents and automatic so the do not need to opt in themselves.