How common are side effect from Cefdinir antibiotic for child?

Most children do not. have any side effects from Cefdinir (generic for Omnicef). However if there are, the most common is loose stools. A rash, including hives, can also occur, especially if the child is allergic to other drugs in that class, called cephalosporins. Some children who are allergic to penicillin are also allergic to that class, but most of the time they are not.