1) Not all do and you should not rely on it because your child might not like it anyway. Eventhough it is overnight, there is no guarantee your little one will sleep. I bought some small toys and activities and I brought out something new for each hour. I had bought a handheld game, but my son was not interested in it. They did have an inflight movie for children that he liked, but if he has a favorite, then a portable dvd might be good. For the trip I bought an mp4 player and put music and shorter videos on it that he liked (actually, it also had games). Mostly, he liked coloring and frequent trips to the lavatory to play with stuff. So, it really just depends upon your child and what they like to do.
2) I do not have the CARES system, but you do not need it. I believe that most airplane seats were designed for 6 year olds anyway.
3) Definately bring your own food. You can check out airlinemeals.net to see actual pictures of inflight meals. My son sometimes liked it, sometimes preferred mine, and sometimes ate his own thing. As you know, however, avoid liquids.
Lastly, as another mom pointed out - comfort is the key. We also travel with pajamas, a blanket, and a pillow. My son also has his own inexpensive digital camera to capture the vacation from his perspective. He really liked taking pictures, but if you do that be prepared to be able to download the images or buy a secondary memory card.
Most important, have a great time!
C.