Apple and Google already ship OSes with comprehensive APIs and parental controls. There's not even any porn on the iOS App Store by policy.
Creating liability for random OS and app developers is absurd, and foreign porn websites aren't going to comply with this anyway.
If your child needs a helmet to use the internet, as the politicians announcing HR8250 seem to think[1], Apple or whomever is free to offer that as a feature. There is no need for this to be legislated, especially when the legislation does not work in open source environments.
[1] Not hyperbole. They said that. It was an analogy, but one that highlights how ignorant of the technology the authors of these bills are.