Whatt o do if my ex-husband does nothing to put our children in immediate danger but makes very poor choices concerning their health and well-being?

You can petition the court to modify custody orders whenever it is in the "best interest of the child(ren)."  The standard does not require that the child be in danger, only that the modification is best for them.  If they are at risk because of his poor choices concerning their health and well-being, then yes, you would have a good chance of getting custody in court.  Before you file, make sure that you have your evidence together to prove the effect of his decision making on the children.  Trying to gather evidence after filing tends to make modification requests more clumsy an ineffective.