These days, there are numerous resources available for offering mindfulness to children.
One book that several parents have spoken highly of is Sitting Still Like a Frog, published by Shambhala Publications. Teachers and parents may really appreciate Awakening Joy for Kids, by James Baraz and Michele Lilyanna.
There is a great Australian website, called Smiling Mind which offer good meditation resources for children. Annaka Harris has a website with guided meditations for children age 6-10, which could be helpful too.
Plenty more can be found by googling “mindfulness for children.”