Be quiet Akriad and let the 9 year old who's repeatedly saved your asses save your asses one more time.

I wonder if Nilha engineered this situation to be able to play with the machine.

And the weaker-than-necessary barrier means the darkness will break through and the kids will be desperate to agree to whatever she suggests next. But what would that be?

The thing is, she could totally have extorted more or less anything out of the kids, on pain of letting the darkness eat them. But she's being nice about it. Uncharacteristically nice, if Zalander's commentary is anything to go by. I think she's looking for some kind of long-term alliance.

My best guess for now is, Nilha is one of the many beings trying to "slip through the cracks" into the "real world." And maybe that requires hiding inside a human host while the temple activates.