The mother's actions can probably be explained with temporary insanity or even perhaps manipulation by some creature. But the father seems like he just took off with the other kid for his own sake without any other context to work with here. Even if he was intending to come back with some solution though, it's not like he could have made it in time with the two of them both attracting all kinds of shit. He would've known something going wrong wasn't even just a possibility without him being there to help ward the things off, but extremely likely to be exactly how things go down.