A source I found (I've replaced some words with "teen" or "family member")
WHAT TO TELL A YOUNG CHILD?
This will depend on their age and where they are in their emotional, social, and cognitive development. There are no formal guidelines on what is best to tell a child and when, so use your best judgment and knowledge of the child.
It might be tempting to avoid saying something altogether with a very young child, since it can seem that they are unaware of the issue. However, children are often more observant than adults realize and frequently see what is going on, even if they cannot verbalize it. Acknowledging that something is going on with their loved one’s behaviour can validate their emotions and help them feel more safe and secure.
Some brief statements that might help very young children understand an eating disorder in a friend or family member:
“Teen has trouble eating properly. We are all working together to help her learn to eat better to get healthy. We all need food to help our bodies function properly.”
“Teen gets very frightened and upset at mealtimes. THIS IS NOT YOUR FAULT. We hope that this will get easier for them with treatment.”
“Having an eating problem can be very scary. It can also be scary when a family member has an eating problem. Do you have any questions about eating problems? Is there anything you would like to talk about?”
“Your family member's eating problem is being helped by grown-ups and doctors. Your job is to be yourself.”
“Your family member is not choosing to act this way. This isn’t their fault. It’s not your fault. We’re all doing the best we can to help them get better.”