Before introducing your dog to a child, tell his parents that you're working on training your dog and ask them if they'd like to be a part of it. If so, toss them a high-value treat (e.g., a piece of chicken) and have them hand it to their child to give to your pooch. Let the child know not to touch your doggie since they don't know each other yet. Keep your pup on leash a few feet away from the child, ask for a sit and then let your pup know to go over and meet the child by using a cue word like "okay." Let the child know to put the hand with chicken out and let the doggie take some chicken. Keep the first meeting short and gradually build up if it goes well.
When you're introducing your dog to other pets, always have them meet on neutral ground if possible; I recommend going on a walk for first meeting. Allow small amounts of sniffing but keep your adopted dog on leash in case there are any uncomfortable situations. Slowly and gradually over time, you will be able to increase the length of time the animals are together and eventually, if all goes well, without your presence. It may take awhile for animals that don't know one another to establish a relationship.
If your new adopted dog wants to meet another dog on a walk, use the greeting ritual mentioned above, however without the use of chicken (so no fighting occurs for the food!). Allow the dogs to sniff each other on loose leashes, and if you see any hard posturing or stares, calmly leave the encounter and keep walking your dog.
Choose to practice proper introductions with as many new pets and children as you safely can, so that your dog can practice this new routine. Make sure to always set your animals up to succeed!
Break your puppy's bad habits for good!