What I always tell my patients (and their parents) when they come in with a concussion is that it is a "brain injury," and --like you would for any other injury in your body--it needs time to rest and heal. Otherwise, each successive concussion increases your chances for another one, and these effects accumulate over time. Also, a second injury before the brain has healed can possibly result in a fatal condition known as "Second Impact Syndrome."
Keep in mind that in children, symptoms of a concussion may be harder to decipher. I always get concerned when I hear a Mom or Dad simply say "He's just not behaving right." Other symptoms include headaches, nausea, vomiting, sluggish behavior or confusion, mood changes (ie: a normally stoic child crying or laughing excessively), or pain with loud noises or bright lights.
If you feel that your child has any of the above symptoms, then you should have your child checked out by their doctor. They'll give you information on what to avoid (physical activity, bright lights, cell phone/texting/video gaming use), and how to know when they're starting to improve.