Also, because children can be unpredictable and often don't eat everything they're given, parents often wait till after the child's finished eating and then give the insulin, to avoid overdosing and causing a hypo. This is really the case when the child is using fast acting insulin eg Novorapid to bolus for food, and especially when they're carbohydrate counting, so that the parent can work out the dose to be given according to the amount of carbs the child has actually swallowed.
Sue