Depend if society provide any social support to single parents. Here in Sweden I think there is a strong argument to say that children here are not worse off if they happen to have a single parent that get support from the state compared to a child that have single parent with support from the state and child support from a second parent. The point of the Swedish social welfare model is that all children has the same possibility in life regardless of the wealth of the parents, which mostly seems to be true for the less wealthy half of the population.