Preventing children from using E2EE is not a "net positive" just as preventing adults from using E2EE is also not a "net positive". Privacy is a human right, not a right that should be made exclusive to adults. Just because children are restricted from other activities does not make it justified to restrict their rights further.
End-to-end encryption helps prevent messages from being intercepted by intermediaries, such as companies and governments. For example, an advocate (for human rights, environment, or any other cause) who is under 18 may wish to coordinate a protest or some other activity while minimizing exposure to surveillance from potential adversaries. Or in a more ordinary example, children may want to communicate with others without being profiled by advertisers or government agencies through the content of their messages. Secure communications are as valuable to children as they are to adults.