Not a bad movie, but what was up with Kate?

The stranger or Jane Bryant ( her name in the novel) was a humanoid form who went about leaving changelings. Her animal totem was the Cuckoo bird, so both she and her offspring will follow the ways of the cuckoo and do 'anything' to survive. Heed the initial conversation between her and the Marlow parents when they ask how many children she has, and she says quite a few...then she makes a remark about it's not always a question of wanting, is it?

Getting back to the subject of changelings . Changelings were fairy babies that fairies left to live with human parents, only Bonnie was no dream... She was the penultimate nightmare. Also take note of the cuckoo bird. It lays its egg in a neighboring nest and its hatchling assassinates the rightful hatchlings solely to get the best of everything from the adoptive parents.

Note that Bonnie is physically stronger than the dad. She bites his fingers off and pulverizes him when he comes after her for revenge when she bumps poor Lucy off.

Sorry to be so lengthy, but notice the flapping of wings at the beginning of the film during the runs cape across the field in the intro. She'd been watching the family for some time. She saw that the Marlow were excellent parents and wanted Bonnie to have the best parents.

As far as Kate, Kate was the ultimate mother and nurturer. Her strong love would never allow her to stand by and let any child face harm. Jane Bryant knew this. Too she might have been under an evil spell of sorts.