A controversial law banning parents from smacking their children has been passed by the New Zealand parliament. Prior to the new law, parents could use "reasonable force" to discipline their child. However, the new bill provides police with ample discretion over whether to prosecute offending parents.
Proponents of the bill believe it closes a loophole in child abuse cases where parents claim abusive disciplinary practices were "reasonable" and not illegal. Opponents claim that it will criminalize parents who are simply trying to control their children.
I am not aware of any law in Iowa that specifically prohibits physical punishment; however, if the discipline goes beyond the bounds of decency, or reasonableness, the Department of Human Services has jurisdiction to investigate the same and has the subsequent power to remove children from abusive homes. Of course, if the punishment causes intentional injury you may be looking at a child endangerment or other similar charge.