aggression towards other kids

Question
I have an American bulldog, he will be 2 this June. We have had since he was 12 weeks old. there is my husband, myself my 12 year old daughter and 10 year old son. just lately he has shown aggression towards kids that come to the door. barking growling even snapping once the door is open, at first i thought is was an off day for him as i was upset at the time. but now he shows it everytime. it takes for ever to calm him down. we got a muzzle from him after the second showing of aggression to wards another friend of my daughters whom he never had a problem with before. We started using the muzzle everytime he goes out side including the back yard as my nextdoor neighbor does day care and the dog with uncontrollably bark at them through the fence with all hackles up. when anyone comes over he has this put on now inside the house. until he has calmed down ( only works if they are not kids) he has even showed this behavior towards my 1 year old niece that was at the time very quiet and in her mothers arms he too had his muzzle on. we now remove the dog from the environment IE downstairs if the kids have friends over as he can not be trusted with them. he shows no aggression to wards my kids at all. what can we do to curve this situation and eliminate this behavior. as now if a kid knocks on the door i don't even open it, i call the kids to the other door to talk to their friends while i distract the dog. or he will charge the door. can you help?

Answer
First thing I would do is make sure there are no underlying health concerns. Because you are seeing this issue with children, I would suggest you ask you DVM for a trainer referral or search on-line for local trainers that would be able to work with you and your dog. In this situation, it is difficult for me to give you direction when I am not able to observe what is happening.

Good luck!

http://www.webvet.com/main/2008/08/18/dog-aggression-toward-kids
http://leerburg.com/aggresiv.htm
http://www.hilltopanimalhospital.com/dominance%20aggression1.htm