B.M.
I know this is not super close to Brookline,but there are classes at ON THE MAT, yoga studio in Concord MA. You can read about them on their website, onthematyoga.com
Here is the blurb from the website:
**Tara's Childrens Yoga classes
Thursdays 3:45-4:45 ages 4-7 and 5-6pm ages 7-11. Both classes are have a parent child option, $14 drop-in per child, $130 for ten class pass, which can be shared amongst friends and siblings and has no expiration date. Parent child option cost is $22 for one parent and one child or $200 for a ten class parent child pass. This pass cannot be used for two children.