Yes, you need separate reservations. It’s easiest if you book the kid on the companion ticket reservation with an adult so you don’t have to convince the system that the kid is not an unaccompanied minor.

However, if you do book the kid on a separate reservation (I do that if one ticket is award and I want the adults to be the ones accruing frequent flyer miles, for example), book the adult ticket first. Then when you book the kid’s ticket, towards the end of the booking process, there’s an option to provide the record locator for the adults to tell the system that the kid is travelling with adults. It’s actually quite slick, something I haven’t seen on other airlines.