[header] How to know if your child is too skinny [title] Weigh your child regularly. [step] A good sign your child is is healthy and not too skinny is if they are steadily gaining weight as they grow. Weighing your child regularly can help you calculate things such as body mass index (bmi) as well as ensure your child is adding weight as they get taller. 
[substeps] Use a scale that is properly calibrated. You may even want one that can weigh to fractions of a pound this will give you the most accurate reading of your child's weight.