One of the easiest ways to practice mindfulness with your children is sensory awareness. Put simply, paying attention to ones senses is practicing mindfulness. What you hear, see, touch or taste all count. For children, this is totally within their capabilities. If you are able to train yourself and your children to concentrate on senses, concentration skills improve overall!
Mindfulness and children is not a new thing. There was a study done with pre-schoolers over 12 weeks that showed mindfulness exercises helped boost them in a number of categories, not to mention empathy, academics and mental acuity. See more about the study here