In my opinion, parents who take their underaged children to a bookstore should also assume the responsibility for supervising them. If you don't want your child reading adult-oriented materials, that is certainly your privilege. But at least pony up some responsibility, and not try to parlay your child's presence into an excuse to deny real adults to read materials that they would like.
Leave the rest of us alone. Don't try to impose your values on people who have not asked for them.