Children not having any effect on them seeing their parents in an open relationship that they never were before, says to me that those parents and children did not have a good relationship.
It SHOULD affect them. The fact that it isn't, tells me what a joke this polyamory crap is. They've not thought this through, they're basically making memes and innuendos canon. They have no idea how to handle this properly and they're not even attempting it(not that it's really possible with these characters, but holy hell, even most fanfictions have better development than this).