Parents who are divorced need to budget for those payments. It doesn't matter which side of the matter you are on. If you are making those payments, you must ensure that the money is available. If you are the recipient, you should set your budget to use them as a windfall just in case they are late.
Some parents balk at the idea of having to pay child support, but this isn't something that should be seen as a negative action. Children are expensive. There isn't any reason why one parent should be left having to cover all of the costs of raising a child that it took two parents to make. We know that child support is sometimes a touchy subject, but it is one that must be addressed.