Teaching kids how to shoot foul shots is an easy concept for the most part. However, free throws are often underrated in terms of importance as they are extremely hard to master. Fundamentals are important, but having the right mental approach is just as significant which is why good shooters can sometimes struggle at the free throw line.
Start by teaching the fundamentals of shooting. The kids probably have a general idea of what to do at this point but have them practice regularly and remind them of the proper technique. Their shooting hand should do the work and their guide hand should just guide. Also, their legs should help with the shooting motion, they should come down and then up as they release the ball.
Inform the kids about free throw rules. Their body should be in line with the basket and their toes should be just at the edge of the free throw line. Also let them know they have only 10 seconds to shoot in most instances.
Tell them they can shoot closer to the basket at first, if need be. Younger kids may find the distance form the basket to the line may be a bit much for them to accurately practice the fundamentals. At this point, mastering the proper form is more important than shooting from the same distance as the professionals. Teach the kids to sharpen their mental focus when they are at the free throw line. Some opposing players may employ subtle distraction techniques such as clapping their hands or coughing when the kid tries to shoot. Tell the kids that no matter how high pressure the situation, they should relax and envision themselves and the basket and stick to form.