Does your 2 year old know when they steal the cookie and they’re lying? Yes, they do.
Do 4 year olds know the difference between roleplaying and reality? Yes, they do. Basic child psychology class stuff.
Does reality and fiction sometimes get confusing? Yes, throughout our entire lives.
It’s not to say that people don’t have areas of development – but the basics are in place pretty early. Would these girls have done this were they in their 30s? 50s? 7?
Likely? Yes. They’re wrapped up into each other and trying to see what they can get away with. Not _all_ kids are psychopaths, even though there are some brain development things that keep going to the age of 25… and some reality distortions, stereotyping, etc.
They were flexing their “what can I get away with?” muscles. Combine with excessive self-assuredness, undersocialized or underhobbied and looking for trouble out of boredom.