Many gay conversations amongst the young inevitably have several questions at the top of the list. When did you come out? and to who? Situation with out to family members questions are often the follow up. 
Of course, responses run the gamut. Of families informed, responses also run the gamut. Often kids don't out themselves to family while of school age. Usually, it is because they fear a bad parental reaction. This law is about respecting those children's instincts. To permit disclosure to parents will keep a number of children away from GSAs, and I suspect those will be the very children that could use a friendly family of some sort, the most.