It is better not to use weight alone to decide whether a child is overweight as some children are heavy because they are simply big while others are heavy because they are fat. Usually the body mass index for age (i.e. weight in kg/height in m2) is used to decide whether a child is overweight due to excess fat. Special charts are used to define the normal body mass index for age. A body mass index for age between the 5th and 85th centiles is usually regarded as normal.
Even body mass index cannot always differentiate between obesity and a high lean mass.