That doesn't actually fix the issue. If Facebook is trying to set itself up like Chrome with the webplatform, or GTalk with XMPP, then they will drive the feature set of ActivityPub, whether you're federated with them or not.
Hypothetical example:
Want to see this picture/video from someone on Threads? You need Facebook's proprietary picture format, which has DRM baked in it. Even if you don't federate, Mastodon, Lemmy, etc now have to take energy away from their work to adopt the proprietary picture format. It depends on the proportion Threads takes on the network and how they can leverage that position to put pressure.
Threads currently has voice notes. Should all ActivityPub services support that? If so, do we adhere to Threads' standard or not?
Another argument to give your tween a smartphone is that they need to learn how to use it, to develop a healthy relationship with it, to understand the pros/cons, to understand how to use it effectively. Abstinence will just make them envious and less likely to think through the consequences.