I think kefir is great! Though actually I make probiotic yogurt at home out of raw goats milk and prefer the flavor of that better. If you listen to the pro-kefir folks, they'll have you believing that kefir is so much better than yogurt. I'm not convinced about that. But I think it is a great way to get probiotics into your intestinal tract which helps your digestive system. This page is the best source for kefir info: http://users.chariot.net.au/~dna/kefirpage.html
I give some to my 20 mo. old DD and she does not have any problems with it. So possibly you can try a bit with your children and see if they can tolerate it or not. I actually prefer giving fermented milk products to my child over non-fermented because the fermentation breaks down the lactose and makes the proteins more digestible.
ETA: kefir is different from drinkable yogurt b/c there are different beasties in it. The bacteria/yeasts in kefir cause fizziness from the fermentation.