Yoga, as well as meditation, can be a beneficial discipline for children. As much as for adults. But I must warn you that little children may not sit still for very long. So use your parental intuition and decide what is best for your family.
Some of the benefits of yoga for kids are:

• Ability to concentrate
• Better ability for children to follow instructions
• A method for them to release stress and other energies
• After a time they become more aware of the power of their mind — this can lead to a better attitude overall

Below are some links to different poses that are appropriate for children.

• http://www.childrensyoga.com/tryyoga.htm
• http://parenting.ivillage.com/gs/gssports/0,,9jv56vdk,00.html
• http://www.parents.com/fun/games-printables/indoor-activities/yoga-for-kids/