Although Bailey rips the twins names out of Marco's book, he doesn't remove anyone else. All of the other performers are stuck in the circus until they find some way to die like Tara. The Murphy twins' parents had to watch their kids grow old and die while they stayed the same age.
Isobel was able to take her name out of the book and leave, so presumably the other performers could ask permission to leave if they really wanted to.
Bailey ADDS his own name to the twins' page and puts THAT page and no others in the second bonfire. It's ONLY Bailey and the twins that are bound to the circus. The epilogue in which Poppet is a young woman despite Bailey now having an email address confirms this.
Tsukiko mentions that no children have been born to the circus since the twins, even though some of the members get it on often enough that it's unlikely for there never to have been any pregnancies. Then later Marco mentions that the other performers aren't actually stuck in the state they were in when they joined up, they're just aging very slowly. Just how many of these women might unknowingly be pregnant with zygotes that will take decades to grow? For that matter, if any of the performers were menstruating on the night of the first show how long did it take them for their period to finally end? (Which may actually be a good question to know the answer to, if we wanted to work out exactly how much their bodily processes have been slowed down.)
It at least is unlikely that there was prolonged menstruation going on. So much bleeding would surely not go unremarked upon.
Since a period is the body's response to an event (namely, the closing of the window of opportunity for the egg presently in the uterus to be fertilized and implant) and not the event itself, its length probably wouldn't be affected by the slowed aging process.
If Marco has the power to slow down everyone's aging to such a significant degree, he probably has the ability to make sure that no one gets pregnant.
When A. H. comes to takes Marco away from the foster house, Marco goes with him, even though A.H. is cold, snappy and refuses to tell anything about himself. Yet Marco preferred the company of such person to being in a foster house.
The physical book itself is almost completely black and white, even with black edges on the pages. Then there's the red ribbon place holder. Before reading the book it just looks like a pretty finishing touch but after reading about how the rêveurs dress it all makes perfect sense.
There's the color scheme in general, for that matter: black and white, with splashes of red. Throughout the book, Prospero and A.H. try to sort the matter of which of their schools of magic is better in a black and white manner. But like the red that breaks up the division of colors, Celia and Marcus Take a Third Option and find that collaborating their magic makes even greater results, proving that it's impossible to simply rank one type of magic over the other.
Widget is born just before midnight, and sees the past. Poppet is born just after midnight, and sees the future.
Widget's conversation with Mr. A. H- near the end of the book reveals that Widget is the narrator of the story, made clear when his story for Mr. A. H- begins with the first lines of the book. However, the first lines of The Night Circus occur in one of those second-person chapters in-between parts of the book, and those sections are established as taking place in the future when the very last one of them includes Bailey's email address. Widget begins to recount the story to Mr. A. H- in 1903; e-mail did not exist yet, and since Widget sees the past and not the future, he'd have no idea that it would ever exist. As such, he can't be the narrator of the book.
Unless Mr. A. H- later added those parts in himself.
Or Poppet writes them.
The thing is, they can't be. Poppet is in London when Widget begins his story; and A. H- is the audience, hearing those words from Widget firsthand: "The circus arrives without warning". How could either of them, at that moment, be putting the words into Widget's mouth? And why would they? Thought perhaps the issue should be restated as this: Widget is the narrator, but him being such creates a minor plothole.