Question about aggression!

I have an american bulldog, he was the alpha male out of the litter and I enter him in ring sport competitions. He is Socialized and does well around strangers unless giving a command, but at night time he goes into guard duty. I can not explain it but its like someone flipped the switch, he will not let anyone except our family in the house! I can live with that but what bothers me is he will sleep in our childrens room at night and if me or my wife walk in there in the middle of the night he will come flying of the bed growling and snapping until he realizes who it is but even then he will continue to growl for another 2 or 3 minutes until he finaly calms down. We have begun to call his name before we enter the room and then he knows it is one of us, but even then he will lay right by the kids and growl at us for a few minutes! I have talked to the breeder and he says its the american bulldog nature to be protective at night as they was often used as farm dogs and I have also talked with my trainer who said he has seen alpha males guard shoes and laundry from a wife when the man was gone but never anything like this. So his advice was to not let the dog sleep in the kids room anymore. Well we tried that and he becomes destructive, he will chew through a door to get back to the kids, if we chain him up he will bark untill morning and nobody sleeps! we are reaching for the stars now, sorry for going on but I need some answers and suggestions, should we just stay out of HIS KIDS ROOM AT NIGHT...LOL? Do you think this is a anxiety problem? An aggression problem? Is it something we should have to worry about? He has never bit one of us but has come real close a few times until he realizes who it is!