What are the reported adverse effects of gabapentin used in children?
Limited literature data, suggest that gabapentin may cause rash that is severe enough to necessitate discontinuation in a small percentage of children.
In a large survey of all age groups: The commonest adverse effects seen were somnolence, fainting, ataxia, nystagmus, tremor and headache, fatigue. However, their incidence was low and intensity mild. In a pediatric group only somnolence and dizziness were reported in 2 out of 33 patients.
Behavioural adverse effects are more common  in children with intellectual disability and attention deficit, worse in <10yo :  hyperactivity, defiance, irritability, agitation, aggression, explosive outbursts, oppositional behavior, often  warranting discontinuation of the medication.