dog suddenly aggressive

Question
i adopted a mixed breed dog a year ago from the kennels i work at. I had another dog already and they got along great. My other dog recently passed away and since then it seems like my current dog has become aggressive with every dog he comes in contact with. "benjie" got along fine with other dogs i never had any problems with him but now if i walk outside with him and theres a dog anywhere near him he freaks out and starts lunging towards them. Even the dogs he was previously ok with he starts growling and barking at. I always keep him on a leash but its getting to the point where i cant leave him outside on his tie out to use the bathroom without worrying that one of the neighbors dogs(who are never tied up) will wander into the yard and he'll attack them. Also if anyone i know brings a child over he is real aggressive with them and starts to attack. I never crate trained him so i've just been keeping him in another room, but i want him to be able to be out with everyone without having to worry about him. He's only 30 pounds but he is alot to handle. I dont want to hurt him in any way trying to keep him from dogs if he comes in contact with them i just want some help. I understand that i dont know much about his past, i dont even know what he is. he was estimated to be 2 when i got him a year ago so now he's 3. I also tryed to bring another dog home so he wouldn't be lonely but that didn't work out at all. any help would be appreciated

Answer
It's possible the older dog (who died) was a stabilizing factor for the other; sometimes a confident dog will reduce fear responses in a less confident dog.  Tying any dog outdoors is not a good idea for a lot of reasons, one of them being that such a dog cannot follow his instincts (flight) and cannot interact normally (due to restraint).  It's not uncommon for a dog that is routinely tied out in an area where other animals, adults and children have free access, to develop unwanted responses to those stimuli, including aggression.  To continue to subject the dog to such stimuli, knowing that he's developing a serious problem (toward children) is not wise.

The lunging you describe (toward other dogs) may not be aggression; some dogs become over excited and react inappropriately at the sight of other dogs.  However, because the dog has been tied, he may very well be acquiring an aggressive response.  Regarding children, dogs need to be socialized to all sorts of people, places, things and other animals from very early in their lives through the second year, something that obviously wasn't done for this dog by his former owner.  You know that this dog is "real aggressive with them and starts to attack" and this is not likely to be rehabilitated; further, it's a clear and present danger to children.  This sort of aggression (from your description) needs serious evaluation by a professional.  I suggest you locate a certified applied animal behaviorist in your area from this site:

http://www.animalbehavior.org/ABSAppliedBehavior/caab-directory

Without in person evaluation and observation, I have no idea what you're dealing with; you may be misperceiving events (optimally), but if the dog really does have a serious aggression problem toward children you need in person professional help.  I can't ethically address that in this venue.