How long has your child been experiencing this? Differentials to be ruled out include: absence seizure, mental retardation, autism or attention-deficit/hyperactivity disorder.
Absence seizure involves only a brief, sudden lapse of conscious activity. This may look like the child is merely staring into space for a few seconds. Signs of absence seizures may include: staring, lip smacking, fluttering eyelids, chewing, or hand movements. Absence seizures last only a few seconds. Full recovery is almost instantaneous. Afterward, there is no confusion, but also no memory of the incident. True absence seizures cannot be interrupted by voice or touch. They also often occur right in the middle of a child's conversation or physical activity.
For a proper diagnosis, it would be best to have your child evaluated by your doctor. A detailed description of your child's symptom; blood tests which can help rule out other potential causes of seizures; EEG or brain scan may help with ruling out seizure disorders and other causes.
Take care and keep us posted.