Meditation for children is against the very nature of being a child. Stillness is not a part of their intrinsic nature.
However, you can make it a regular practice by practicing it with them. I have been teaching mindfulness to kids and as a parent have been practicing for 2 years now. We have a bed time routine.
We have a singing bowl and we take turns in closing our eyes and doing mindful listening. We also do breathing meditation with our stuffed toys. I have smaller kids and so I use stuffed toys. With older kids its way easier.
We do mindful eating once a week where we just focus on texture and mindful walking in the park. Teach them young and as they inculcate this habit, it will become second nature to them.