One of the biggest questions that I have had in raising my children is whether or not to teach them meditation. And the answer that I have reached is that when they have the attention span to sit still for more than five minutes is most definitely yes.
But there are some guidelines for you to follow, if you decide to teach your children how to meditate.

1. Don’t expect too much from them. Remember that they are just kids
2. Remember also that they do have a short attention span. Don't force them if they get bored or antsy.
3. Never show disappointment or impatience with children.
4. Make it perfectly clear that it is not a competition. Especially if you have more than one child.
5. Keep all instructions simple and use as few words as possible.
6. Use a gentle and soft voice when giving those instructions.
7. Show a good example. If they see you being patient and, caring, happy and relaxed then they are more likely to be and stay motivated.

When you teach meditation don't focus so much on having them clear their mind. Instead have them focus on a certain positive emotion or on a syllable or a picture. In an earlier post I talked about making mandalas. If your kid makes a mandala then they can use that to meditate on.
Here are some links on meditation for children. I encourage you to explore those sites and use the information their.

http://www.buddhanet.net/metta_k.htm
http://www.susankramer.com/ChildMeditation.html