Both my kids have been fighting off a head cold. My oldest for about a week and my 3 year old for…

Both my kids have been fighting off a head cold. My oldest for about a week and my 3 year old for going on 3 days. My oldest is almost back to normal but my 3 year old doesn’t seem too be getting better. Going on day 3 for him and NOW he has started up a low grade fever. Cough and mucus pretty bad still too. I’m wondering when would be a good time to take him to the doctor. Is it too soon? I don’t want to be sent home because I brought him in too early being that it’s only been 3 days and the fever started today