score:6
According to the Ryanair website this is possible:
Short summary:
You have to book an "adult" ticket, then contact Ryanair via chat or phone to link the bookings (free within 24 hours after booking the second ticket, a fee is necessary later). On the day of travel, you have to check in online, then go to the counter at the airport in order for your child to check in (also free of charge).