I am sure the parents already know. Do they go to the pediatrician for well visits? They get weighed there too. My kid was way below the percentile chart for years. Was on meds for acid reflux. Then at age 3 decided it's just a case of being petite. And a picky eater. There's not much I can do as a parent. Unless this family is extremely uneducated and poor, they are well aware of their child's weight. Maybe if this is an area where you think the family does not go to a pediatrician once a year, and really needs an intervention as far as feeding them, then maybe give them a call. But as a parent of an underweight child, there is no need to. Would I be mad if you called me? No, I would understand that you are concerned. But don't be surprised if the parent says yes, I know, and there is nothing we can do about it.