I’d read about safe worlds, inside snow globes that sat on shelves in little girls' bedrooms. I’d read about safe worlds— inside snow globes that sat on shelves in little girls' bedrooms. I’d read about safe worlds inside snow globes that sat on shelves in little girls' bedrooms. I wanted to read the sentence with a pause there, but maybe I'm too close to the sentence to be objective. In the first two it reads: safe worlds ... then goes on to describe a particular safe world: safe worlds [like those] inside snow globes. Maybe that's what I need: safe worlds like those inside snow globes. In the third option it reads: safe worlds inside snow globes. Opinions?
If you're trying to be visually striking, and slow down the readers pace, the second one is the best option.
I feel like you might need even MORE of a break than the em-dash. Maybe something like... I’d read about safe worlds—you could find them inside snow globes on shelves in little girls' bedrooms. Without the really obvious shift, I feel like it gets a bit confusing - was the narrator inside a snow globe as she was reading, etc. Not bad enough to REALLY slow me down, but enough to be less-than-perfect.
So you'd go with "safe worlds like those inside snow globes"? Or do you mean even more of a break than that?
It depends on the larger context, obviously, but if you're trying to set up the reader to be hit by the narrator's cynicism, I really like the em-dash. It's like the pause before a punch line. Or a punch, in this case.
I prefer the comma or the third option depending on meaning. The em dash reads too much like a paranthetical aside, and I don't think that's the effect you're going for (I could be wrong). Also, if you do use the first two options, I would leave out the "inside." It's functioning like an adjective there, and that's probably not what you want because it's a bit unclear.
Third option for me - however, without the pause it's a little long and becomes confusing - like I could read the sentence without really getting its meaning. Adding "like those" as you suggested does fix that, imo.
Call me crazy but I'd maybe lengthen it by chopping it in two sentences and echoing the idea of the safe world by attaching it to the girls bedroom. Because you have a kinda interesting nesting doll visual here of a safe world contained in a snowglobe in the seemingly safe world of the little girls bedroom.