I'm not sure if single child households have done this to parenting culture as much as neurotic culture/economic incentives have pushed single child households. When everyone is competing it makes sense to focus on one child as you don't want your child to be at a disadvantage vs those who can spend on tutors/extra curriculars/.... It's a problem in Italy and some eastern countries, a bad and anti-social evolution in my opinion but I doubt it's going to change.