As I understand it, this is NOT a case of a child travelling unaccompanied, but a case of a child travelling with an adult, but on a separate booking.
As I said before, I think that the hurdle at which this arrangement will fall is the online check-in process. The system will have no way of knowing that the child is not unaccompanied, and will not let the child check in.
Maybe the way round this is for the child to check in at the airport (and pay the check-in fee).
Maybe there is another way round this, such as getting the child added to the adult's booking manually.
Maybe the easiest solution would be for the adult to make a fresh booking for the two of them together.
Lots of maybes... I think the only thing the OP can do is contact Ryanair for a definitive answer.