I taught theater to that age group for six years. I switched parts on occasion, but usually because it worked well for all involved. Sometimes there is a good reason to switch a part, although in the situation you describe, it seems hard to justify.
I think you, or someone, should talk to the scout leader, and try and understand her rationale. The fact that one girl suddenly wanted a bigger part after a week is not usually a good reason to take it away from another girl. If the leader wanted them to audition for the parts, she should have done it from the beginning.
Since the girls wrote the play, I think they should stick with the original choices. It's not like they are putting on a major performance.
It's possible the leader just thought the other girl was better in the role, and wanted an excuse to change them. I know I have often preferred someone else in a role, but I would never take a role away from a kid, once I have given it to them.
Talk to the scout leader -- she should tell the second girl that she realized it was unfair to take the role away from the other girl, and she should give the part back to her.