sure, but plenty of software already exists for those devices to block adult content and social media. it works just fine without a header. its actually even better, because that software can even block nefarious websites that would never comply with adding a header
Blocklists are useful, but a hint from the website that, actually, they don't want to cater to children would be useful when those blocklists aren't up to date.
A locked down iPhone or Chromebook is going to thwart everyone but the most determined without compromising any privacy.