dog aggression

Question
My daughter adopted a part Pomerianian/Collie?mixed breed dog from the Humane Society 8 months ago, it is now 1 1/2 years old.  The dog was aggressive towards her and her two small children for no reason.  The kids would walk up to pet it and it would snap at them.  Most of the time the dog was ok and pleasant, except for these unexpected, unprovoked attacks.  They were planning on putting the dog to death last Monday, so I said no, I will take her and see what I can do with her.  I thought maybe it was them that was the problem.  The dog is extremely hyper and jumps a lot.  I had pretty good control within the first hour of her arrival at my house.  I have two Scottish Terriers as well. The second day she was sleeping and one of my dogs went over to sniff her and she immediately, viciously attacked.  I stepped in and stopped it quickly.  I thought it was just a dog thing so didn't worry over much about it.  Last night I spread a cover over the sofa and my two Scotties were sitting beside me, and I wanted her to feel like she was part of the family so I invited her up as well.  They all went to sleep.  About midnight I got up, and got down on my knees beside the sofa to tell her it was time to get up and go to her cage.  I was petting her and talking softly to her when all of a sudden she visiously attacked me.  I was stunned and it scared me a little.  Now I am thinking maybe they were right and that she should be put down.  I spoke to my vet this morning and she said some dogs are just plain aggresive for no reason, and that she would not keep it.  I have very small grandchildren, and I sure do not want this dog attacking them.  Do you have any idea what might cause a dog to act like this? We are in a dog training class, but all the training in the world cannot stop an attack if you don't know when to expect it.  

Answer
If this dog were being eval'd by the humane society and it exhibited these behaviors it would not be offered for adoption.
It is up to you whether you are willing to work with a dog like this.  It could be the way she was born into this work but I suspect it is the way she was raised or allowed to be before your family got her.
I wouldn't take a chance with a dog like this with children.

-Trish