You really can't. In developing countries having more children is always a net plus. They don't take up much more room in the house (or shack, let's be real here) and can be worked like little slaves with no oversight to help produce wages for the family. Plus it's fairly probable that some might die in infancy, so families overshoot to make sure they still have enough children. Five children, ten children, fifteen children… for most families it doesn't matter how many they have, because they pay for themselves quickly and use up little additional resources. Even after they grow up, societal mores mean that money will still come back to the parents.
In developed countries, children are always a financial burden. Socially speaking, they're going to need their own bedroom, so you have to up your house size. They will need to be educated which costs a lot of money, and cannot contribute jack shit until the age of 16 with the exception of some chores. Currently, it costs about $250,000 to raise a single child to the age of 18 in a developed country, and the parents WILL. NEVER. GET. THAT MONEY. BACK. For almost everyone in the middle-class, having more than two children is simply financially impossible. Could you afford an extra $30K per year? I thought not. For some couples, having even one is something they have to put on the back-burner until later in life when they have stable careers and are homeowners instead of renters. If 25 becomes 35 becomes 45 before that happens… some will just decide not to bother.