We performed an updated review of the available literature on weight gain and increase of body mass index (BMI) among children and adolescents treated with antipsychotic medications. A PubMed search was conducted specifying the following MeSH terms: (antipsychotic agents) hedged with (weight gain) or (body mass index). We selected 127 reports, including 71 intervention trials, 42 observational studies and 14 literature reviews. Second-generation antipsychotics (SGAs), in comparison with first-generation antipsychotics, are associated with a greater risk for antipsychotic-induced weight gain although this oversimplification should be clarified by distinguishing across different antipsychotic drugs. Among SGAs, olanzapine appears to cause the most significant weight gain, while ziprasidone seems to cause the least. Antipsychotic-induced BMI increase appears to remain regardless of the specific psychotropic co-treatment. Children and adolescents seem to be at a greater risk than adults for antipsychotic-induced weight gain; and the younger the child, the higher the risk. Genetic or environmental factors related to antipsychotic-induced weight gain among children and adolescents are mostly unknown, although certain genetic factors related to serotonin receptors or hormones such as leptin, adiponectin or melanocortin may be involved. Strategies to reduce this antipsychotic side effect include switching to another antipsychotic drug, lowering the dosage or initiating treatment with metformin or topiramate, as well as non-pharmacological interventions. Future research should avoid some methodological limitations such as not accounting for age- and sex-adjusted BMI (zBMI), small sample size, short period of treatment, great heterogeneity of diagnoses and confounding by indication.