This is a common topic on a board I participate in. This is a board full of mostly very frequent fliers and their take on it is that you need to be responsible for your own needs and not expect strangers to accommodate you.
They say that there are cost/benefits to many seats on an airplane and it is an individuals responsibility to make seat choices that best meet their needs. If you can't stand to look out a window, book the window seat so you have control over the shade. Same thing if you want to look out the window...book the window seat so you can have the shade open.
If you end up not getting a window seat and want the shade closed, then you need to bring along eye shades and not expect the person at the window to close the shade to meet your needs. It could be that the person in the window seat gets severely motion sick if they can't see out the window and if they end up not booking a window seat, then they need to figure out what medication works best to help with that and not expect the person in the window seat to open the shade for them.
Generally, the only time the shades are required to be open are during take-off and landing per the flight attendants request. On red-eye flights, the flight attendants will usually ask you to lower the shade so that the light doesn't disturb other people who want to sleep. This is because when flying trans-atlantic or trans-pacific it can be common to fly when it is still light outside and frequent fliers usually try to sleep if it is nighttime in the country they are traveling to in order to help with jet lag when they arrive.
Having said the above, it is also perfectly okay to ask politely for the shade to be opened/shut as long as you are willing to accept a no answer. I generally try the best I can to accommodate others, but I know that when I am asking a favor of another person, they definitely do have the right to tell me no.
As for moving so parents/children can sit together...this is a hot topic and generally people will only trade if they are getting a better or equivalent seat. Nowadays, many people are paying for specific seat assignments and they are unlikely to want to trade down especially since the seat they chose cost them money.
I do know that just taking someone's seat and then asking them to trade is the sort of thing that can cause tempers to flare. The term used is "seat poacher" and it tends to get a strong negative reaction from frequent fliers. Much better to hover and not settle into the seat that is assigned to another person and then politely ask them if they would be willing to trade. And accept that they can say no and their reasons for doing so may well be private.
I remember on one flight a woman got upset because the man in the aisle seat wouldn't trade with her for the window seat. It escalated and ultimately the woman was reseated at the back of the plane. The gentleman she was demanding to switch seats with was very polite, but quite firm in that he would not be able to trade seats.
It turned out (we got to talking during the flight) that he needed to use the bathroom about every half an hour and had back problems that made it difficult for him to get out of a window or middle seat that often. None of that was apparent from him just sitting in his seat and he didn't want to share his private medical concern with the angry woman who wanted him to trade seats.
Mostly, I try to remember when I am flying that it is public transportation and that people are often stressed and nervous. Trying to be kind to others can make a miserable flight a much more positive experience for everyone.
