From the
Ryanair FAQs...
CHILDREN, INFANTS & YOUNG PERSONS
Ryanair does not carry unaccompanied minors under 16 years. Children under the age of 16 years must always be accompanied by a passenger over 16 years. Escort and special facilities are NOT available.
[...]
There's nothing there I can see about having to book a child as a child, or having to book everyone (i.e. child plus accompanying adult) on the same booking.
No idea what might happen at online check-in stage, as
Aviatrix mentions above.