For an event I need to have a responsible adult on site, but there may be two or three (grandparents often attend), I need to collect their details and have them volunteer for certain parts of the events. There is no cost for these parents attending, but they may purchase t-shirts etc.
The kids then have a whole different set of questions and costs
I can see how a pre-booked event can help, but I want to add up all the entries into one invoice.
You can create a custom field so the user can select Parent or Child. based on their selection, the price can be modified (up or down). We're about to release 2.6.6 later today that adds a brand new option for custom fields... where you can create conditional fields. Then you could have additional custom fields that show after a user selects Parent.... and a whole different set of custom fields that show if the user selects Child.
The only setback is that you can't REQUIRE that there is at least one parent registering. You can of course give that in the instructions, event description, etc... but there would be no automated check to verify at least one parent was included in the registration.
Yes, you could make a parent event, then a child event... make the parent one a prerequisite... definitely. However, they would be two separate transactions. Support for registering multiple events in one payment transaction is in progress, but not yet available.