You can teach children Wise Reflection at any age. You have to be creative when teaching children about the laws of existence. Instead of formally sitting down, which is quite boring for children; you have to make it creative. For instance, you can go for a walk in the park and get them to imagine themselves as an ant, imagine themselves as a butterfly, imagine themselves as a snail. This is getting closer, without actually mentioning Dukkha, which is something most little kids won't want to hear about. Remember, they are very sensitive..
There is a story about a six-year-old who's part of a family which has two children. One night there were only three pieces of fruit for two adults, and two children, so one of the adults said, "Ok, I don't need one. You can have the others. One for each." But the six-year-old frowned and said, "No, no that's wrong!" He raised up from the table and grabbed a pencil and a piece of paper. On the paper, he drew three circles and four stick people. Then he started to draw lines in the fruit. He raised his brow, and his face lit up and he said, "I have it! If you cut the fruit this way, everybody gets an equal amount and no one has to go without!" How many of us can be that generous? Remember, this is a six-year-old.
One time the those same adults had to go on a long trip, and left their children with friends. When they came back, the friends said to the parents, "Your children are not normal." The parents responded, "What do you mean they are not normal?" And the friends said, "Well, I was walking in the garden with your three-year-old one day and I wanted to pick one of the flowers. But your three-year-old said, "No, no, no, don't pick the flowers. It's fruit for the bees and the birds. You can enjoy the flowers without having to make them yours."
So don't limit a child's potential. They can learn. It's just a matter of how much time we spend with them, how creative our mind is, and how much we really want to share our knowledge with them. That is to share our knowledge, not just teach it to them. With that, comes a whole different attitude, sharing our knowledge by communicating with them.
Young kids can also learned to do short periods of formal meditation, especially Lovingkindness meditation, during which they focused more on the happiness and peace side of the meditation, wishing for others to have peace and contentment. Five or ten minutes might be enough. Don't expect them to sit down for half an hour or more, because usually children don't have the patience.