Thanking the sun offends people? Really!? Give me a freaking break... I get that these are kids, so this makes things a little tricky, but come on.
Yoga does not have to be spiritual. It is up to the individual performing the poses to make it spiritual or not. And the great thing about it, is that if you want it to be spiritual you don't have to make it Hinduism spiritual. I am Wiccan, and I have had at least one deeply moving spiritual moment while doing yoga. If these parents have an issue with the spiritual nature of Yoga, they can tell their kids to think about baby Jesus while doing their poses, it's not that difficult of a problem.