Ryanair Prime bookings can include infants (1 infant per adult on the booking to a maximum of 2), however, infants are excluded from the Ryanair Prime benefits.
Minors (2-15 years) cannot be added to an existing adult’s booking (as they are not allowed to travel alone and are not a Ryanair Prime member), however you can create a new adult booking for them and link it to your original booking so you can travel together. Note that the minor must travel accompanied by an adult aged 18 or over. The passenger type will be updated for each minor during the linking process to either a child (2-11years) or teen (12-15 years). For more information on linking bookings, see here.