Won't happen because WN (unlike other airlines) empowers its GAs to solve problems. In this case it makes eminent sense for the child to board with the parent (which - as others have pointed out - is exactly what happens on other carriers).

In addtion I think the whole spouse/companion business is much ado about nothing. If WN doesn't allow family to accompany the person with the highest boarding #, they will simply save seats. I have never seen anyone get in a fight over a saved seat. I also find it strange that someone would want the saved seat, which is almost always a middle seat. No doubt people try to save both the middle and window, but these days, with most of my flights at 100% capacity, you aren't going to have that luxury.