There are a few places which i came across where they teach yoga for kids
There is a Raja yoga center, where they conduct workshop for kids. it is located in karama.
There is another one called Bliss yoga center.
Then there is Ayuryoga http://www.ayuryogauae.com/
I am not very sure about some of the locations.
You can find more about yoga classes here : http://gulfstay.com/blog/yoga-classes-dubai