You will not be able to book minors online. They system will not allow it, unless they are on a reservation with someone who is at least 18 years. You will have to call Delta to book those. After that the reservations cannot be combined into one reservation PNR. They will cross referance all the reservations so that you won't have to pay Unaccompanied minor fees. However the minors will also not be able to check in online, they will have to get their boarding passes at the airport. No work around for that either.