You will need the Full Write Forms permission to perform this action.
Both People and Repeating Section fields can be connected to existing item/ticket fields on a form. This means you can have it so that each person or repeating item added during a submission will automatically increase the number of tickets chosen/purchased.
You will need to add the ticket field before you will see the option to link a ticket to a People or Repeating Section field. Click here for more info on the ticket field.
Example
If there is a $20 cost for each child being registered and 3 children are registered, the form will automatically choose the associated tickets and charge the expected $60 without requiring the submitter to choose those tickets manually. This works for free tickets as well.
Below is an example of a form that has a People field along with a connected item/ticket field. The number of breakfast and lunch tickets is automatically changed based on the number of children being registered.
Note: the user can add additional tickets if desired, but they won't be able to go below the number of children being registered.
