My 12 year old child is doing a 3-day fast, just drinking water. Will she get sick?

Not likely. I don't think this is a good idea since a 12 year old is still developing. What I would recommend is picking a special day a week for the number of days, say 3, then fast on those single days not in a row. I also suggest lemon water which is frequently used, no sugar added. Much better and the real point is showing dedication.