Actually, it is. A reasonably well-off family has more time to spend with their children, to help with school work, etc. They have more money for learning supplies (both learning toys and things related to school). They can make sure their children are nutritionally fed (not that they always do, but they have the capacity and it's more likely). The children are less likely to need to start work early in order to help support the family.

In short, being a child in a well off family results in that child being more likely to be better off later in life.