Event registration allows free registrations (but shouldn't)
A way to do this would be the following:


Enable in the Event Configuration the option "Register multiple participants?"
Use in the priceset a radio button for child/adult. You can make it required and add a different price for each.


You will have the option to set a different "Profile for Additional Participants", so you can collect different data for the person entering the form compared to the other participants.

That way you know who will attend the course and know per registrant whether it is a child or parent.