I don't really have much to add except to second all of the wishes that it hadn't gotten explained at the end. I pictured a closing scene of the groundskeeper coming in to carefully repaint the line as the "camera" pulls slowly out and fades to black. I kind of prefer that one.
For most of it, though, I was deeply enthused and enjoyably creeped out. Very young viewpoints are difficult, but rewarding when they come off, and this one was mostly a hit in that respect. The tone of childhood was very strong, from the egregious use of last names to the complex and arbitrary Rules outlining the common-sensical reactions one naturally has. (Of course walking around it doesn't count, and being pushed, and etc. etc. etc., but to kids, those all have to be Rules, not just assumptions, and it ends up feeling arbitrary and oddly baroque for what amounts to a simple concept. Very true to children, in some ways. I approve.)
(P.S. - Where were all of you "It's better left unexplained" commenters on "Terrible Lizard King," huh? :-P There, everyone wanted to know if the dinosaur was "real" or not. No pleasing you people, I swear.)
(I'm kidding in that last bit there. No offense or actual umbrage on my part should be inferred.)