Relationship between recognition of unhealthy food brands, among children, and their BMI.

Hello, I am writing a research proposal titled, Relationship between recognition of unhealthy food brands, among children, and their BMI.
I was wondering what are the main ethical concerns regarding this type of research? I am aware parental consent would have to be obtained. But mainly focusing on the ethical concerns of measuring a child’s BMI; if the child is willing to participate and the results are anonymous Thank you