It is important you look at the things your child likes to do.
Taking lots of different things doesn't harm, because during long crosses they need to have some variation.
I would advise not to take to many electronics, but also other things, like games, music, craft stuff, Lego or Play Mobil in order to keep your child busy.
In the interview with Judith Reiff, she explains what they took along for their children
Games were one of our favorites. Especially “kolonisten van catan” (the best -Dutch- game ever!) was very popular. During crosses we played “kolonisten” at least ones a day.
When playing we set an alarm for 15 minutes. Why? Well it takes a little longer than 15 minutes for a boat at the horizon to come close enough to hit you. So one of us had to go out and take a look to see whether there were boats. In the meanwhile someone else finished his or her turn.
The game board stayed on the table, and put plastic desk mats over it when we were not playing. It stayed on the table, even when we were eating. In this way it is easy to start a new game.
We were lucky we had a catamaran. With a monohull it could be different. But you will find out yourselves what works best for you.
In my room we had bags with Play Mobil and Lego. My sister and I loved to play with the Play Mobil together.
When we were with other children we usually played in our boat, because we have more space on board. While the parents were in the tub, for happy hour, we played inside with the Lego. We took all the Lego into the living room and spread it out on the table.
We had three laptops on board. One for navigation and two for other stuff.
Besides school work I kept a dairy, wrote an English story and watched movies with my sister on the laptop. But watching movies stopped after a while, because we rather be outside.
We didn't have TV onboard. If sometimes, we met other people who had one, me and my sister would be a little jealous. But I also realized we didn't have the time to watch much TV since we were busy living outdoors.
Together with my sister I we had a Nintendo ds on which we played games and listened to music. During the trip this also faded a little. When we met children who had a Nintendo too we grabbed ours but besides that we didn't use them much. But the Nintendo was very useful when I wanted to do something for myself.
I remember playing with the Lego and Play Mobil more than using the Nintendo, because playing with other children was simply more fun.
My sister and I loved to craft. Under the benches of the living room, there was a lot of craft stuff. While making our artworks we listened to music and sang along.
We used the watercolor paint. To paint you need to mix paint with water. We made paintings for most of the people we met during the trip.
Even if your children aren't creative, it's good to take something along. My sister isn't creative either but she did enjoy doing it onboard. Also other children we met liked to craft with us, because they couldn't do it on their boat.