You can do what you want via groups. I would turn off the default filter (set to unfiltered), create two groups; kids - adults; and for kids, add the kids as users, and adults as the adult users.
For each group, configure content filtering accordingly.
Select users for group by clicking on them.
Users are configured in webconfig; be sure to enable proxy service per user.
Requires non-transparent proxy and user authentication.
Based on my understanding and brief experience of content filtering, this should work.
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