It all boils down to each individual kid's personality, I think. Some children are naturally more aware of their surroundings than others. Some children are comfortable with the idea of "taking the stage" and being seen. Other children are pretty shy and don't like to jostle for position. Some are just so precise in following what they were told the first time that they are paralyzed when asked to make spacing adjustments as necessary during the show.
One thing I have learned over the years is that I have to be very conscious about giving blocking and movement directions. Some directors like to create scenes and choreography that sort of grow over time in a very flexible, fluid and ever-changing fashion. Young children really need to have set movements from the get-go. They get confused easily.
Crowd scenes on stage are the worst. The obvious solution is to ask the taller people to stand in the back. Or when a lot of actors are on stage, many directors ask the children to find a "window" between the heads of the shorter people in front of them and stand in it. And yet, that is an elusive concept for some.
PLANES - Imagine grid lines on the floor of the stage. They are the imaginary horizontal or vertical lines you may direct the actors to position themselves in.
PROXIMITY - is the distance between the individual actors. They must always try to maintain some "daylight" between characters and not get too bunched up.
LEVELS - the heights either of individual actors or the heights when they are positioned on set pieces or risers. The actors may also modify their heights by kneeling, bending, lunging, or kneeling, holding their arms up, etc.
Game Play - Begin music and ask the children to dance or move like an animal around the stage. (You choose the animal for each round such as dinosaur, bird, horse, kitten, butterfly, etc. Having evocative music helps...) After a few measures, stop the music. The children freeze, and look around and try adjust themselves to make a good stage picture. If they are a tall person in front, they must make themselves shorter. If they are short and in back, they must make themselves look as tall as possible. And so forth. They should never block the people behind them and they should adjust positions as needed to provide "daylight" between actors. Repeat as many times as desired. Caution - Don't overdo it!