I am not 100% sure it is available in English, but have a look at KiSi (Salfeld).

Imho the best parental control app - while it is not free it is run by a small German company (I had very delightful contact with the founder), falls under the GDPR, offers very flexible scenarios, a windows client that offers roaming, various options to lock down the internet (whitelist/blacklist both manually or curated), reporting (what webpages are consumed, which are blocked, what apps are used how long). Times can be set per group and both by the time of the day and by usages times (e.g. this group can always be used, this can only be used during the day and this group can only be used weekday afternoons and weekend all day but only for 2h per week across the PC and the smartphone). It also allows requesting access/more usage times via a "parents Version" of the app.

It's not self hosted, it currently does not support Linux, but after testing basically everything on the market I accepted that one. Funny enough now eldest has asked for the already massively reduced access(no social media, only limited Youtube, mainly wikipedia and similar sites whitelisted) to be blocked during certain hours of the day (and their PC access blocked as well - which is a different beast as we use FreeIPA and Fedora) so they have 90min of focus time for homework.