Facebook is cancer. Letting this play out will only condition kids be more immersed/addicted to social media. How can that be good for them now? How can that be good for them in the future? Facebook has proven that they toy with its users, their psyche, their privacy.
This will definitely come to hurt them in the future. Either by FB selling out every 1_and_0 they got on you, or allowing 'by accident' another scammy CambridgeAnalyticaV2 to siphon all your data, or by getting hacked and disclosing that 2 years late.
Take for example needle exchanges. People who run needle exchanges are not asserting that people being addicted to heroin is a good outcome, only that people being addicted to heroin and also having to share dirty needles is worse.
Likewise, I'm not asserting that little kids using Facebook products is a good outcome, only that it may be less bad than other outcomes.
I'm not a parent, but I imagine controlling your kids electronic device use is a difficult task. The jump to them having a smartphone must be like a dam being blown apart with dynamite.