What a question! I was asked this (anonymously) by an 11 year old.
Luckily I have the time with my class to do a unit of work about self esteem and social conditioning so I think the answer to this question lies in the understanding that beauty is down to social conditioning-it is not a universal truth, and we can prove that by looking at different cultures and their beauty ideals alongside historical beauty ideals in our own countries and how that changes over time. Therefore it is not a fixed or absolute notion.
We also talk about the effects of the pressures of living up to beauty standards such as mental health problems/eating disorders/body dysmorphia/obsessive behaviour around fitness etc.
Then I would explore who it is exactly that benefits from us feeling like we should be skinny all the time? Where is the messaging coming from? What’s in it for the beauty and fashion industries to keep us chasing after different beauty ideals….MONEY!! What’s in it for influencers and stars to promote a beauty ideal…MONEY/FAME=MONEY!
I might also go into how if we are all busy and concerned with our own looks and changing them all the time, then the greatest beneficiaries of that is government and/or the patriarchy because we are then not concerning ourselves with the bigger issues such as understanding policies/challenging them or the status quo etc. But that might be a conversation for older children!