If you look at the Ryanair website under the Family Extra page, it says this:
"Free seats for kids
Get free reserved seats for up to 4 kids aged between 2 and 12 with every adult seat booked. As part of our Always Getting Better programme it's now mandatory for adults to have a reserved seat when travelling with kids, this means you don't need to worry about being separated on board."
That's as clear as it could be.