The low-cost carrier says the change, which Will come into force from 1 September, is to prevent Young Families from being split up on busy planes and delaying departures.
Although adults will have to stump up and pay an £8 premium for their plane seat, their children's reservations will be free of charge.
In a statement, the company said it had to introduce mandatory reservations because many adults travelling with young children were not choosing their preferred seats in advance.
This was causing boarding issues as crews attempted to re-seat customers with young families, not least because Ryanair's flights this summer are said to be running at 94% capacity.
In some cases, it was also causing customers who had purchased a reserved seat to be displaced.
Kenny Jacobs, Ryanair's chief marketing officer, said the change will allow families to "fly safe in the knowledge that they are getting Europe's lowest air fares while ensuring they always sit with their children".
However, it could leave some families disgruntled as the Irish operator is already notorious for extra fees - some of which catch out flyers at the last minute.