Surrogacy and Parenting

NY Times article entitled "No Stork Involved, but Mom and Dad Had Help." This article is about the challenges of answering the question "Where did I come from?" when the parents have used a surrogate mother to get pregnant. What would be the best way to talk to a child about surrogacy (given their developmental level)? Should parents entering into surrogacy think about how to tell their child before even entering into these contracts?