Third party services to validate age are ridiculous. Add and RTA header [1] on the server side and create laws to put the liability on the client to look for said header and be done with it. Not perfect, nothing is. Teens will get around it but most small children will not which is the spirit of such measures. Congress critters should support using the RTA header as their own browsing habits will otherwise be leaked or sold and their behavior will be outed using third party verification systems.
[1] - https://www.rtalabel.org/index.php?content=howtofaq#single