On most of the Ryanair flights I've been on, only a small handful of people use the priority booking system. (There are often none at all). So you'll probably have the whole plane to choose your seats from. Just make sure that you don't occupy the seats adjacent to the exits, as airline safety rules prohibit their use by children. (If you did sit there, the cabin crew might not spot your error until everyone else had boarded, leaving you to find the few remaining seats elsewhere).
Anyway, as Bednobs indicates, people are usually happy to change seats to allow family groups to sit together.
Ryanair keeps its prices low by maximising the time that its aircraft are in the sky, and minimizing their time on the ground. They work to a 25 minute 'turn around'. That means that, once an aircraft arrives at a gate, they've got just 25 minutes to get all of the passengers and their luggage off, carry out any essential cleaning, restock all of the food and gift trolleys, get the new passengers and their luggage on, refuel the aircraft and complete the pre-flight safety checks. Such a tight schedule simply doesn't allow sufficient time for passengers to look for reserved seats.
Chris