Day 1 - Yoga

With the help of Itsy Bitsy Yoga our very first activity was Yoga!
K loves doing the mountain pose,

the downward facing dog and the child pose.

R just liked to watch!
I feel that Yoga for Kids doesn't have to be as regimented as some would argue. Learning to breathe is a great way to calm temper tantrums, and promote a calming and peaceful day.
With all activity's you have to make sure that your kids are open to the experience, so I would suggest doing Yoga bright and early in the morning or late at night just before bed. This is when they are most receptive.
There are lots of classes that teach yoga for kids, such as Growing Up, ItsyBitsy, Yoga4KidsCalgary, StoryTime Yoga, but doing the moves at home can really bond parent and child.
A great place to start would be a Yoga book as described above or Yoga Cards can be great too!
Get down on their level and just have fun with it, and maybe you can work it into your routine for a more balanced you.
Namaste.