For our retreats, our minimum age is 20. The retreats are far too intensive for kids five to seven or, we feel, even 18 or 19, but it is possible to teach kids meditation. It has to be in a soft way. It has to be in a gentle way; nothing super-heavy. We would not advise teaching the death reflection to little kids. It's just outside of their ability to understand. Talking about the word Dukkha and the unsatisfactory nature of life, impermanence and all these high-level Buddhist terms -- no, most kids can't understand that very well.
But kids can develop in ways that they see their parents develop. If the parent is more patient, the kid will be more patient (in general, not always). You can teach them formal meditation with simple meditation subjects. Maybe put a flower down in front of them. They look at the flower, they close their eyes, they try to see the flower. They can do little concentration practices like this. They can do a little bit of the Compassion/Lovingkindness meditation. You sit with them and say, "Let's think of Mommy. Let's think of nice thoughts for Mommy. Now, let's think of Grandma. Let's think of nice thoughts for Grandma." A simple way of practice. They can learn and they're going to learn a lot by watching you as parents.