That is not the first time that Interrail.eu reservation service has this problems with Trenitalia reservations. For most train companies your are adult from about 14.
If you are traveling now, go to an Trenitalia Ticket office or “Last Minute” Desk in big stations and ask for changing of the reservation into 2nd class, if there are free seats left, you will get for free the change, if you start in a station without office, ask before boarding the train staff.
If you have time till traveling, than ask directly interrail support and I will also tag @Nanja . Maybe you could also ask directly Trenitalia via Social Media if they can help you, pleas ask them with DM on Twitter or Facebook and provide them the PNR code that you got on the reservation.