I have not been able to do online checkin for Ryanair flights with my under-2year old, so I presume you've paid your €10 per person checkin fee?
Assuming you have paid for checked luggage, if you feel you are approaching your baggage limit, stick the cases on a weighing scales before leaving and take stuff out if the cases are over. You will be charged if you are at all over - no discussion.
Paying for extra things with Ryanair is not much fun. Make sure that you have paid all your little fees and charges before you go to avoid having to halt going to the gate while queuing at Ryanair customer service to pay the extra charges.
Go early, early, early. There's a lot of hanging around, but airports and the excitement of travel are interesting for youngsters, plus you'll be thanking yourselves that you aren't in a panic to make it in time.
I've gone through security recently with bottles of water for the child and other things that I thought they might pull me up on, but they did not. Don't assume its okay to carry all sorts through tho - I've heard various reports good and bad about instances where people were let or not let carry liquids etc., through security.
Ryanair do not call passengers with children to board first. I believe the priority boarding thing is a take on, so just be ready to board promptly so that you can get seated together. People won't push you out of the way, but you won't be treated any differently to other passengers. Ryanair flights tend to be quite full too.
Try and bring your biscuits and treats for the children yourself. The onboard selection is very limited and expensive.
Finally, Ryanair have recently changed their coffee supplier, so once you're in the air and everything's done and dusted, get yourselves a cup of 'parents' aid' and it'll taste not too bad!