If you think that he has croup, you should probably (at the very least) keep him home. Croup can be contagious, so you wouldn't want to spread it around his daycare. If he doesn't improve within a couple days, or if he's constantly coughing, or having trouble breathing, you should probably get him seen by his doctor.
He sounds like a seal barking when he coughs?? You can take him into the bathroom and turn the shower on as hot as it will go and let him breathe in the steam. You can also give him some coffee (if he will drink it) that can help too. If he still feels bad in the morning... take him to his Dr.