Not advisable. The courts always have the authority to determine custody and child support matters and will ultimately determine what is in the best interest of the children. If you were to define custody or child support matters in a prenup, circumstances will necessarily be different should you ever separate due to the passage of time. Therefore, the terms in any prenup would almost certainly not be enforced by a court (as courts can alter these terms even in a separation agreement if the parties later change their mind).
