A billion dollar fine is entirely unnecessary to dissuade a typical driver from speeding. The fine needs to be enough to change the behavior, not 500,000,000% that amount.
Meta is worth $1.5T while most of us getting tickets are worth less than $200K. Meta brings in $200B/yr. Taking 10% of that, even 20%, would hardly slow the company down in most of its business.
I think 15% of revenue per year of uncorrected behavior is about the right amount. After a few years of that, I'd wager Meta would start to change how it treats children, probably by screwing over adults even more, but that's fine with me. Adults get what they deserve. Children deserve better than being handed over to a mega-corporation for monetization.