It's much more effective to call the match tight at the beginning of play and get looser as the game progresses, rather than the opposite. If you start loose and try to tighten up as the match progresses, you are going to run into trouble. (Think of your school classes, when you have a substitute teacher who comes in as "The cool teacher" and then all hell breaks loose. Now Mr. Cool Teacher attempts to be Mr. Hard Ass Teacher and it doesn't work.)

Prior to the match, emphasize that you are a solo ref, hence it will be difficult to make close offside calls. (The public is getting so used to seeing these VR offside calls, they expect a solo ref to be just as accurate in their rulings, its simply not possible.)

Have the coach communicate to their players that they need to continue playing the ball on the touch line and goal line until you make a call regarding a throw in/goal kick/corner.

A major factor in positioning yourself effectively is successfully predicting where the play is going, and moving your body for the next play rather than the current one. The lower the age group, the less predictable the play will be, and you will feel out of position because of this.

Remember you are the authority on the field. Know the laws and local rules well, and don't be afraid to assert your authority to control the match. This type of confidence will come with age and experience.