Brittany Ferry Advice for a total novice

It's fine, to be honest you will probably end up being dragged around the different decks and inside and outside.

As per the ship guide www.brittany-ferries.co.uk/ships/cruise-ferries/normandie/onboard-dining-and-bars#tab-3 sounds like the cafe near the soft play area would make most sense for you to try and base yourself. There are plenty of unreserved seats and benches around the different floors. It will go relatively quickly I'm sure

We did the same crossing last year and didn't have a cabin or seat. We really didn't need it! we had a 2 year old so we spent the day walking around the boat, did soft play, went to the bar area where they had entertainment for kids on, had some breakfast, then some lunch later. There was no shortage of somewhere to sit at all. We didn't need to base ourselves anywhere.

You can usually book a cabin onboard. Go straight to the Info desk when you get on. Well, after the other adult has found 4 seats in a cafe or somewhere.

I think you will be fine. Just don't take too much stuff out of the car as you will be walking up and down lots of steps with whatever you take throughout the crossing. When you get on find out about bookings for the cinema and restaurant if you want either. The main restaurant can be good value with younger children as the children's meal prices are very cheap and they also get a BF goody bag at the end. Your kids will probably enjoy being up on deck as the ship leaves (and at the end when it arrives in Caen). In the summer hols they usually put on children's entertainment so you could sit in the café nearest that. I would not worry about trying to keep one particular table as everyone else will be moving around so it is likely you will find somewhere to sit when you need to. The restaurant will probably be oversubscribed so if you want that you will need to queue at the allocated time.