I'm not a lawyer, but does the law require 100% un-hackable accuracy? Not the law as-written, but as-enforced? Has anyone been successfully penalized because they failed to catch one under-13 year old who said "Yes" to a dialog? Most sites I've seen either ask for your birth year or your age, and trust what the user says. If that's enough to be in compliance, why would any company make it more complicated? It's like if you've ever visited an alcohol company's web site, they ask "Are you 21?" and if you say yes, that's all they care about.