I won't call myself an expert in this field, and haven't given it much thought, but a couple options just off the top of my head...

1. Limit child accounts to "classic" social network functionality. They get to see things from mutual friends. No algorithmic feeds, kids aren't in the user search, and no way for messages to be sent/received unless both sides have consented.

2. Disable chat for child accounts. How many chat apps do children really need? Each one is another potential vector for issues that parents would need to monitor.

I'm sure there is a monkey paw here, but either option seems better than no end-to-end encryption for anyone, at a time when government surveillance is a bigger issue than ever.

Frankly, I think option 1 would be better for all users, not just children. Go back to classic "social networks". This "social media" experiment has failed.