They say there are techniques to teach the children to do right. The elders have to be diplomatic though some say they have to be strict. Children will be upset if the elders are hard and will surely not follow anything anymore. If you talk to them nicely, they will be happy to follow:

Socialize with the kids and talk to them lightly. This is to have an opportunity of having the child to open up and telling them what to do. When they become your friend you can tell them that they have to knock before they enter the house or a room. It would be much better than scolding them why they do not knock on the door. Scolding will not be effective.

Tell them that you will give them their favorite snack or a pint of ice cream if they learn how to knock once a week.  There are other ways to challenge them.  Offer them anything you know they will appreciate when they are able to do the challenge.

Children are naturally easy to forget.  Put reminders on the doors to help them remember.  Reminders will develop their sense of awareness.

You can try telling every child in the neighborhood this lesson of knocking in the door first before entering.  This will be passed on from one child to another until it reaches your child.  And because it is a group activity, your child will be encouraged to do the same

Locking the door will oblige the child to knock before entering.  If this will be practiced, the child will learn the rule.