We have 2 great danes and a beagle. We have had absolutely no problems until last night my male dane saw my 12 yr. old and 8 yr. old children wrestling, when my son (8 yr. old) walked away the dog grabbed his arm and shook it like a lion with it's prey. My son was wearing a thick jacket (thank goodness), but the dog still broke the skin. I don't understand why an almost 2 year old dog would just snap for no reason, no warning or anything. Never shown aggression or anything close to it.
Right from the start we've had this dog very socialized, always around children and he's very well trained. We were advised to put the dog down by a veterinarian, but I find that a little harsh.
Anything that could possibly trigger a dog that's very passive to do something like this? Brain disorder or anything? I haven't heard of it with danes before.

I am very suspicious of anybody that claims to know what is going on in those furry heads. A dog's personality continues to develop until about 3. As they age, they try to take more responsibility and compete for status. I am sure this relates to the wrestling, but not exactly how the dog sees it. The dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by
Reading some books or going to a good obedience class. A good obedience class
Or book is about you being top dog, not about rewarding standard commands with
A treat. Start at http://www.dogsbestfriend.com/. The dog may have just wanted to join in the fun, and been too rough. The dog may have been punishing the 8 year old for challenging the higher status 12 year old or protecting it. The dog may have been piling on, pushing the 8 year old further down the pack order after the 12 year old did. As much as dogs wrestle in play, they should understand we do sometimes too.

In addition to establishing the dog's proper place, you must have a zero tolerance for using any teeth ever on a person, not even in play. The dog must be quickly punished every time. The simple ''Bad dog!'' in a quiet voice that sounds like you mean it is very effective with some dogs. The only physical punishment I would count on being able to use with such a large dog is my last resort, the squirt bottle. Fill it with water and a little vinegar or lemon juice. Give it a squirt in the face as soon as it misbehaves. Dogs hate that. I do too and only use it in desperation. Denying it your attention works great
Too. Turn your back, walk off, whatever. Dogs are very social, and losing
Your company is the worst thing that can happen. Don't forget the praise and
Petting when he finally yields. I don't know if you can save the dog or not. This is my best idea of how.