What do your children do when people come to your home? Do they always come and say Hello? This is something you should introduce to their lives from a very early age. When they are about 4 or 5 years old. By doing this, it will develop their social skills from a very young age. These skills will then come in very handy when they are older.
Every time somebody visits you, before you answer the door, remind your children that they need to stop what they are doing and come and say Hello. Once they are used to doing this, it will become instinctive for them to come and greet your visitors. Not only will this promote good manners, but it will make your guests feel welcome by all the family.
It will encourage your children to be more outward going as they grow up, and it will help them with their communication skills. This will benefit them hugely when they get older, and other people will form a good impression of them. This may help with job interviews or career contacts in the future. It will also make people value the interaction they have with your child.