Scott
The only OS-level Android option is Google FamilyLink, which GrapheneOS could theoretically implement, just as it's implemented Android Auto. These are proprietary apps which require Play Services to have control over the device, which is normally prevented by Graphene's placement of Play Services into the normal app sandbox. However, reviews of the FamilyLink app itself appear to be mixed, and it seems likely it will not be a priority for the GrapheneOS team anyway.
There are other third party options which do not work at the operating system level. My personal recommendation is a service like NextDNS for DNS filtering at both the device level and the router level, and then installing an app like Norton App Lock with device administrator abilities which will prevent a user installing a VPN to bypass DNS filtering, or changing the custom DNS settings. If you want stricter controls and friendlier alerts, perhaps you just need to consider a stock device such as an iPhone for temporary use until they come of age. Also, definitely recognize that none of this is a replacement for limiting & supervising a minor's access to the internet.
Personally, it is frustrating that AOSP does not implement filters at all. Same with ISPs. In 2025, it seems bizarre that most internet users (both children and non-tech-savvy adults) are still using unrestricted and unprotected devices on a totally unfiltered internet.