I think children do appreciate what they have because life is tough out there for kids. Today's children have to deal with parents who are alcoholics, druggies, meth heads, deadbeats and child abusers. Add to that the stress of bullies at school and kids at school who aren't nice. Children do appreciate what they have, it's just that sometimes children grow up too quickly because of life's unfortunate circumstances.
Kids these days are self-absorbed, narcissistic, sociopathic hedonists. They pout when a privilege is revoked due to bad behavior. They have no concept of causality. They think that their actions have no consequences. They honestly believe that life is perfect. They have a passing knowledge of the rest of the world, but it doesn't affect them none, no sir! And these easily-manipulated idiots are my meal ticket. As long as they're self-absorbed idiots who occasionally get together to crusade for something glaringly obvious or entertaining, like the Ice Bucket Challenge, I can convince them that I, or a colleague, would a good choice to put in power. Har har har! Mine is an evil laugh.
Assuming that we are talking about children in wealthy, developed nations, the answer to this question is twofold. Firstly, and most obviously, children in well off countries certainly take for granted the material goods they have become accustomed to. You can see this by the fact that people in the modern world simply believe that wastefulness and extreme consumption are just the way things are. Secondly, people don't really know what it is they have, and therefore they can neither truly appreciate it or reject it. The society we have been saddled with is far more destructive and spiritually disconnecting than we realize.
No, I do not think that most of the kids of today really give the amount of thanks that they should for the things that the are given by their parents today. I think that it would be best if there was a way to show them what they have.
Children these days tend to be spoiled. They definitely do not appreciate what they have. A lot of kids have always lived during a time where the Internet and cell phones are part of every day life. They do not appreciate how these inventions have made life so much easier.
If they are from the middle class or higher, children today are offered a wealth of opportunities to grow and learn. If they have it good, they are not taught how things could be or how others before them had to fight for more. So it is not their fault all the time if they do not appreciate the good things that they have.