He may need a little bit of obedience or behavior training to get him up to where he should be behavior wise. The good news is these classes are quite effective with dogs. They are most generally offered at your local pet store and or veterinarians office for a nominal fee.
I would recommend familiarizing him with the children and people in small doses. Bring friends and children over for a short amount of time and have them give him a favorite snack. (One child or person at a time) Explaining to your friends that he is people shy is a good idea so they will allow him to take his time getting to know them. Let him sniff around and get used to the visitor prior to them petting him. (Think about it if some one walked into your house and immediately reached up and rubbed your head you might get a little hot as well)
You will also want to families him with other dogs, you can do this by taking him to dog parks etc. Make sure you have control over him at all times and he has a muzzle until you can trust him not to attack.
Make sure you keep a water bottle near to gently spray him should he show signs of aggression with the children so he knows the behavior is bad. You can also use change in a bottle to distract him if he starts growling.
Since he has bitten a person and attacked other dogs this is very very serious. Not only can he be euthanized for these actions but you can be charged in criminal as well as civil court for the attacks. You could be forced to pay medical bills, pain and suffering and the cost of replacing some ones dog. I would strongly suggest that he be kept from other people or animals until he is under control. If he is able to leave your property now this will need to stop now. He must be kept on your property and away from people and animals. Any time he is out I would suggest complete control with a harness and a muzzle.
Some vets agree that neutering a dog will help with aggression. This is not always the case but it has been known to work on some dogs. If you plan on keeping the dog this is something you may want to discuss with your vet. There are also medications that can be used. The sooner you can set him up with his vet for a evaluation the better this will be.