As a rule the more wealth a family has, the less likely they are to indulge in activities that require the attention of social services.
-Not being able to feed or clothe a child? Money solves that.
-Wanting a night out without the kids? Have money, get a childminder. Don't have money, leave them home alone.
-Rubbish school in your area? Money means you can move to a better area, or send them to a fee-paying school.
-Kids are bored? Money buys them videogames, takes them on holidays, instead of seeing them roaming the streets making their own "entertainment".
-Money usually means someone is earning it, which provides a different kind of role model when compared to a family where nobody works.
Etc.
It also stems from the differences in a parent's attitude. A middle class person is more likely to come from a stable background, more likely to have had parents who had a job, more likely to have been able to stay in education, more likely to own their own home. The probability is that they will have a better ability to raise a child to a minimum required standard than someone who relies on benefits, relies on a house from the council, comes from a broken home and doesn't work because that would affect their benefits.
There are obviously plenty of bad parents at the higher end of the wealth scale, and plenty of great parents who are on the poverty line. It's all about attitude. But on average, having money - having always had money - means a middle class person is able to raise a child more easily than the poorest.
I'm not saying a class of person is better or worse. Social Services only get involved if a situation is drastically wrong. Having adequate money and being prepared to work hard mean that raising children is less likely to go drastically wrong than having very little money, living at or near the poverty line and viewing benefit payments and council housing as a right rather than a safety net.