I often hear people say that children allready have mindfulness down and that we slowely grow out of it. I'm never sure how to reply to this as yes children live very much in their own world very often but I would personally not call this mindfulness. My eldest will often be playing with something as he is walking along with me to the shops, at that moment he is wrapped up completely in what he is doing and at the same time walking. He often will walk into things or people. In mindfulness he would take the time for his play and really enoy that not distracting himself with walking. I do think that because children are so able to completely focus on one thing it is much easier for them to join in a mindfulness practise, especially if it is done as part of the family.
I have a little mindfulness bell that the kids are allowed to invite, they have to asl for it since it is up high. They know to bow to the bell and to breath 3 times as it is ringing. They know the bell has to be woken up before we invite it. If you have no idea what I am talking about and you have never heard of a mindfulness bel have a listen to this podcast by sister Dang Nghiem, there is also this video cast Both are free on Itunes and as far as I know they are not avalailable anywhere else.
I think the best way to teach is by modeling and involving our children as much as we can in our own practise of mindfulness.
There is a very good website started by monks and nuns from the plum village tradition that is aimed at young families. It is currently available in English, French, Spanish, german and Portuguese.
Some books we have enjoyed here have been "each breath a smile" by Thich Nhat Hanh (Author), Sister Thuc Nghiem (Author), Nguyen Thi Hop (Illustrator), Nguyen Dong (Illustrator). It has been a favourite that gets read on a daily basis for over a year now. The illustrations are lovely and uncomplicated there isn't a lot on each page to look at, just enough to keep your attention for a short while before moving on.
Next up are the zen tails, these books are available as ebooks which I like because you can have a few on your iphone/ereader etc and they are easily available when you unexpectedly have to wait somewhere.
"Pebble in your Pocket" by Thay Thich Nhat Hanh. If you take a look at the mindful kids website you will see snippets of the book there.
For the parents I suggest "Everyday blessings: The inner work of mindfull parenting" by John Kabat-Zinn.
I hope you will find something useful in all of this :)