Harvin, who has a history of chronic migraines, was taken to a hospital in an ambulance.
Childress said the episode began after Harvin looked skyward during a punt return. The coach said Harvin jogged off the field to see team trainers, returned and was watching practice when he fell sick.
The Minneapolis Star Tribune reported Harvin threw up before falling to the ground.
The 5-foot-11, 184-pound, second-year player was checked by medical staff on the field, the Star Tribune said. Children said Harvin appeared stable before leaving for the hospital.
Harvin caught 60 passes for 790 yards and six scores last season when he missed some practices and one game because of severe migraines.