Is a Rust-Oleum painted item safe for infants?
Any paint recently manufactured in the US should be safe, with the exception of expensive specialty paints that you can't purchase in the home center. (i.e., antifouling paints) That being said, the rule of thumb for any finish is that it will take a month to cure even if it dries faster. After thirty days all of the resins should be converted to a stable, solid form.

You should still apply some common sense and supervision. Being generally safe is not the same as safe in every conceivable situation.