i see what you mean, there is obviously a cap for height, your body will prevent you from exceeding a certain point by excreting substances, or transcription factors and other signalling molecules
but to a certain extent you can change it. my paternal grandparents are both roughly 5'5. my uncles are roughly 5'8-5'10, but my father grew 6'2. he drank a liter of milk everyday and worked out a lot.my uncles had a healthy lifestyle but did not go to extreme lengths like drinking a liter of milk every single day.
so even with a healthy lifestyle (with the kids you mentioned who were properly nourished), you might not reach the full extent. for example, children with genetically short statures can be given hgh to help boost their growth, when previosuly they had the genetic confines that you mentioned. before the bone plates fuse, trying to maximize your height is possible. so instead of the 20% being going backwards/negative/stunting height, it can be used to grow further past the genetic potential