No. . .this is mostly a European thing. But frankly, I'm happy to give my kids yogurt! (Not quite the same I know. . .) Kefir is basically fermented milk. . .you can make it at home and yes, it is rancid which I must assume is why it comes in all sorts of flavors.
Personally I would like veer more towards just a plain old healthy diet. Kefir has antibiotic and antifungal properties but frankly, a good diet and healthy habits like washing your hands will keep you from getting sick.
It's really up to you. . .I liken it to buttermilk. I have a friend that swears a nice tall cold glass of buttermilk is the best thing on a hot day and I think buttermilk is just nasty. In any case, if you feel like your child needs a big huge health boost--go for it. . .

just don't taste it again!!!
I suppose that if they