You simply mark the space that will be used for the stage. Your space is very important. If the space is too small the actor's full movements will not be seen. The children may bump into one another or you will not have enough room for your audience. If your space is too big it could have a terrible echo, the children could stand too far apart creating a problem with your stage blocking.
Props are very important in a stage production.
They can be very large and extravagant or very small and simple. Props are not always necessary. Actors can act as though they are holding different objects or sitting on chairs and so on. If you have access to the props needed in the production use them. They can give actors attributes for the characters that they are portraying.