No, passengers who have trips to plan with Ryanair are not allowed to add a child to an already-booked flight ticket. As per the Ryanair child policy, it is suggested by this airline that you can never have the eligibility to add a child to a pre-booked ticket on Ryanair; that simply means a child under the age of 16 can never be allowed to get added to the booked reservation. On the other hand, the policy also suggests that as a traveler, you can make a new and separate reservation for the child if you wish to, and you have to link the bookings accordingly.
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