Tom Cruise and Katie Holmes were able to settle their divorce quickly. Part of this was due to the fact that they had a prenuptial agreement. A prenuptial agreement determines alimony and property distribution in the event of divorce.
But you cannot provide for custody, visitation or child support in a prenuptial agreement. The reason for this is that the court has jurisdiction to determine what is in the best interest of the children based on circumstances at the time of divorce.
Likewise, child support will depend on custody and income at the time of divorce. The parties can agree on custody, visitation and child support in a separation agreement or marital settlement agreement and the court will normally approve such an agreement. These issues related to the child are the ones that Cruise and Holmes had to negotiate and resolve.