6 years and 1 month - rated 1140 in classical chess online

I am pretty sure openings are extremely important in any level of chess. but the benefits become exponentially less and less if you learn deeper and deeper. (e.g ,The first 5 moves will be extremely precious, but grey in 5-10 moves, less and less worth investing in deeper and deeper lines). And also, a player needs to specialize in a few opening rather than learning here and there in all ECO 500 openings.

Also, important to spend in many other areas of chess and dont stacked in one area.

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Is markmax33 the parent? ("... I came here not really knowing what to show him ...") Is there a reason why the parent should not consider showing the child a game from the sort of book that I mentioned?

To teach him good opening principles and to study whole games based on how that opening is played is useful.  But to learn variations of openings takes too much time, when there are many other things to do.  But whatever course you choose, make sure he is having fun.  If kids have fun, the learning follows. Also take him to a club.  Play kids only.

https://www.newinchess.com/media/wysiwyg/product_pdf/7790.pdf

This exactly. There is no need for him to memorize variations at this stage (in my own experience this has more negative effects than positive for kids - this can wait till he gets to 1500-1600 and should only really be taken seriously once he has reached ~1800). He should learn basic opening principles. He should know basic pawn endgames. Try to get him a puzzle book at his level, solve puzzles with him. Let him play regularly and have fun (this is most important).

Have you ever actually studied any of the books you suggest?

...or do you just read the Forward and the Preface, then put the book on a shelf to gather dust?

"... I’ve chosen 1 e4 e5 openings (Open Games) ... Playing Open Games gives you a wonderful opportunity to improve your skills and knowledge of key tactical ideas you need to master in your development as a chess player. Throughout the book you will find many examples (and exercises, to keep you busy!) involving forks, pins, skewers, discovered attacks, discovered checks, double checks, removing defenders and so on. ... As it’s a First Steps book, I’ve tried to avoid encyclopaedic coverage. In any case, you certainly don’t need to remember every single variation and all the notes before playing the opening. Take in the first few moves and the key ideas, and then try it out in your games! …" - GM John Emms (2018)

The majority of the Experts(2000-2100) I have met did not know the opening variation we were in.  They did know how to correctly sacrifice and defended brilliantly in  tough, losing positions.  

Most of the time, neither. By the way, you did not quote a book suggestion. You quoted a reproduction of a comment from an article.

You think that you can expect that kind of success from all lower-level players?

The point is that a lot of experts do not study openings.  As I have told you before, many grandmasters say that one should not study openings till maybe 2400.  Play, lots of play, is more important.  Then study of personal weaknesses.

"... This book is the first volume in a series of manuals designed for players who are building the foundations of their chess knowledge. The reader will receive the necessary basic knowledge in six areas of the game - tactcs, positional play, strategy, the calculation of variations, the opening and the endgame. ... To make the book entertaining and varied, I have mixed up these different areas, ..." - GM Artur Yusupov

I'm not surprised. The higher I get, the more I can come up with a reasonable plan against anything thrown at me. It doesn't mean I don't want to learn new openings, which I do, but it free's me up to study the endgame for now, which, for me, is a more pressing matter.

Not to mention that most experts and masters say one should have some, but limited, opening repertiore and have a good grasp of opening principles.

MR MARMAX33.  This is addressed to you:  A six year old should have fun playing chess.  That is all that matters.  Should you wish---and should he---wish to improve then simply learn chess notation and look for game-losing errors, such as a piece of his moves to an un safe square or his opponent takes a piece, but your son does not take back.  Also the board is im portant for improvement.  Many 1600-1700 do not know the board.  Play a game with him.  Teach him the names of the squares and then simply ask whether they are dark or light.  It will improve his game, when he can correctly name the squares.  You can even find free apps on line that do this.  Hope that helps.

Caesar, you hit the nail on the head:  it is more important to be able to deal with the position in front of you than to know in this variation I should be playing the St. George Attack.  Strong players figure it out.  

Perhaps weak players could benefit from seeing some sample games?

Not at 6.  Just play have fun.

Don't think illustrative games can be fun?

"... Journey to the Chess Kingdom ... is primarily intended for children and has thus been written in a light and captivating style. ..."

https://www.chess.com/blog/Natalia_Pogonina/book-review-quotjourney-to-the-chess-kingdomquot

Chapter eight is mostly a bunch of games.

The problem I have seen with alot of 1500(otb, uscf) players is they get those books, learn about an idea usually played in that opening variation but which is a mistake to play in the position before them.  If they wish to improve, analysis of their own games, learning how to get better at analysis, and studying combinations would make them stronger.

Are you sure that you know which books? Are all books the same?