Yes. Yoga has been used in india at all ages but not in the way americans assume. Yoga is a lifestyle:yoga is the union of the mind and body, a way of thinking, eating & doing. So teaching your child to respect the world, eat from mother nature, respect their body and others, breathing, taking care of self, and doing the positions (asanas) to help bring awareness is great. Just hard core yoga class, no.
Yes. Yoga is a wonderful practice for all ages. Even infants can participate with their parents. Is this the same as yoga when we're older? No, but it can be an effective mind-body practice.