Key ideas to consider
These are general rules and may differ for different social and cultural customs and practices.
Most children learn social rules without being formally taught whereas others may need some help and guidance to get it right.
Some suggestions to start with
When facing someone when you are standing and someone else is standing facing you, you should be at one arm lengths distance away from the other person. You can ask the child to check this by putting their arm out to start with until they are used to this distance. Practice with your family and close friends first
If someone offers to shake your hand, use a firm grip, shake once, look at the person directly, then let your hand go. Your child could practice this with someone you both know well
When sitting next to someone (unless it is fixed seating), it is appropriate to be abou half an arms length away. This can be measured by clasping your hands in your lap and putting your elbows out