I didn't have a good answer. How do you teach someone to have a loving and thankful heart?
One thing I do know: Everyone, children and adults alike, could learn more about gratefulness. Kids model what they see. If they see us appreciating a simple thing such as a beautiful sunset or a family night together playing games, then maybe our own gratefulness will rub off on them. Children will, for the most part, model the actions of the adults in their life. This means we criticize less, complain less, and point out the positives in situations and other people. Simply telling a child they should be grateful for what they have is vague, it doesn't sink in. Lead by example.
It is much easier to be grateful and thankful when things go the way we planned but it's equally important to recognize the blessings of hard or uncertain times. We don't teach our children "we are luckier than someone else". Instead find the lessons learned in those difficult times and use that in other situations. The goal is to teach them to have "grateful eyes" so they begin to see the need before they have to be told.
If you've planned a day at the lake and it happens to rain, don't complain and whine about the yucky weather. Explain how the rain is helpful for the crops and flowers. And this is a perfect time to set up the tent in the living room!
Kids know how to 'put on their best manners in public but often times they don't do it because it makes their heart feel good but they do it to get what they want. How many times has your child or grandchild turned on the charm with the "Pleeeease" when he wants something, or the "sad face" when he doesn't get it? We as adults are not innocent of this either. We do exactly the same thing by sulking or pouting or becoming angry at the situation when it doesn't go our way.
Take the time to mention to your children the things you are grateful for throughout the day - being grateful for living in a house with a backyard, grateful to eat fresh vegetables from the garden, grateful to paint pictures for grandparents, grateful for time away from the television to focus complete attention on each other with no distractions.