What should I think of and prepare when flying with kids?

I found YouTube the best preparation for flying with small children. Before I went on a plane with my kids for the first time, I started viewing a lot of "airplane" video's. We watched this landing in Madeira video over and over. It is where I learned that Airbus airplanes calls the pilots retards just before touching down ;). You should be cautious not showing these "near misses" or other spectacular videos. When we boarded the plane finally my kids had a lot of YouTube "simulator" training and were extremely curious about doing it for real.

Whether or not you should bring child seats, really depends on the carrier. Bringing it on board can be convenient, if you plan to rent a car at your destination, but be prepared to not being allowed to use it on board for multiple reasons. If you are flying a American carrier you should be sure to have a seat recognized by the American authorities or in case of an European carrier you seat should have TuV logo or the like. Some carriers consider child seat a hazard during take of and landing. In the end you might end up storing the seats in the baggage compartment, because the FA denies using them. Happened to me multiple times. Opinions between FA differ within the same carrier. Only last week I had a transfer on top. On the flight into Lisbon it was an issue, on the connecting flight there was no issue what so ever.

Be prepared to ignore a lot of disgruntled fellow passengers. A lot of people really have a strong opinion against (small) children flying. True, a crying or nagging child can be a nuisance, but so is the bragging drunk businessmen who are also sometimes abundant.

It can be beneficial to learn the word for child or children in the local native language, because in some countries people with children board first. Although I like boarding last, but with children boarding first can be beneficial (especially if you are not pre-seated). The disgruntled fellow passenger will make sure not to sit in your proximity. In case you bring your car seats and the FA disagrees there is still room enough to store them in the overhead compartments. I was allowed boarding in Lisbon first, only because I learned that criança is the local word for child and the boarding announcement was in Portuguese first. Luckily, once it was announced in English, I was already standing in the children line. It was brilliant.

If you have a tablet or laptop, load it with some cartoons and you also might have a relaxing flight.

In my case showing a lot of YouTube, really turned out well, I ended up explaining why planes fly while being heavy almost half of the flight. It made them curious about flying and travelling and they wanted to know more.

This may sound a little mean but if you're really concerned with them misbehaving or crying during the descent, one thing my mother used to is give us Dramamine. Granted she was frequently traveling with three of us under ten, and often by herself. And we all had a track record of being difficult travelers, particularly with respect to the ear pain during landing. But people would often tell her how lucky she was that we slept during the whole flight.

I don't have kids yet, but I'm keeping this in mind for when I do.