Thanks for the feedback. I'm glad you're finding the story inviting.
There's no real reason I haven't gone into the kids' past experiences (except for the one of Karen's which obviously has plot importance) except a desire to avoid burdensome backstory. I'm currently writing something which might be a side story or prologue if it works out. It's set a few days before the portal opens and has a weird event in it which is witnessed by others who actually question their lack of panic. (Current opening line "A few days before the portal opened it rained blood in the afternoon. In retrospect it was probably a warning, but at the time it was just another weird thing." This will probably be edited a lot before it hits the site, but it gives a feel for just how weird their lives are when blood rain doesn't even raise an eyebrow.)
Other than that I'm trying to get people more into the character's heads and give a touch more description.
Thanks again, feedback is a wonderful thing.