This is generally a false statement. Children in most modern societies are more of a financial liability. This is one reason why fertility rates in various societies tend to drop as those societies become richer and more modernized.
In more traditional societies, children are an economic asset. They are an asset in part for their labor. In such societies, a child's labor can bring more to the families economic situation than the child takes away in terms of the cost of feeding, clothing, and otherwise keeping the child. Children are also something of an informal old age pension as they are expected to take care of aging parents.
In modernized societies, children do not typically bring in significant economic gains. They do not tend to work. Instead, it costs a lot to educate them and to provide them with all the things they are expected to have. These societies tend to have government safety nets that allow parents to remain independent of their children in their old age.
For these reasons, children in our society today tend to cost more than they give back to the family, thus making them an economic liability.