Well, I'm trying to get my kids (particularily the oldest) to drink kefir. My youngest will try anything, but the oldest won't. She will drink smoothies, but she doesn't alway want a smoothie. The benefit is that the kefir grains produce the lactic acid in the juice, so you still get some good probiotic benefits from drinking it.
The lemon in the red grape juice made it more tart after 2 days. The youngest dd loved it, but the oldest was at school and now it's all gone.
So I will brew up some more tonight. It's good that you can leave it for more than a day or two, that's the only PINB thing with the milk kefir. If I let the milk kefir go to seperation then I get a very curdy tart kefir, and we all like the thicker, creamier milk kefir.