AGRESSIVE BEHAVIOR IN MY PUPPY

Question
We purchased a black lab mix from a shelter.  He was found wandering the streets by a police officer and placed in this shelter for about a month.  He is now 6 months old.  I'm not sure if his behavior is due to what he is mixed with or how he was treated in the shelter, but he is very aggressive (primarily towards me--a woman).  Usually when being corrected, he bares his teeth, growls, snaps, barks, lunges and then bites.  Unfortunately, as we are trying to work with him, his behavior is not improving.  Did I mention that he also appears to have separation anxiety when placed in his cage at night and tries to find ways to escape out of the yard.  I have two children--ages 5 and 10.  For no reason at all he will grab my 5-year-old's arm in his mouth (and bite down fairly hard).  His bad behavior seems to be mostly in the morning.  Therefore, my husband has been taking him for 2 to 4 walks a day to tire him out.  We have contacted two dog trainers and the one suggested that we give him away so that he doesn't hurt the children.  We want to give him a fair chance, and would hate to take him back to the shelter, but we can't sacrifice ourselves and our safety for him.  Please help!!!!

Answer
We are slaughtering millions of dogs every year for lack of homes.  The logical thing is to let him be one of them.  But we have hearts as well as brains.  I understand why you would hate that.  I do not have a proven, successful method to solve his problems.  I am not sure anybody does.