I have also been experiencing the same. I can't see child's application on FamilyActivityPicker, however when i click on a category and add a rule, the rule DOES get enforced on the child's phone. ScreenTime API is really inconsistent in its behaviour and has given developers so much pain.

We have the same issue and it really sucks so much that apple simply won't fix these trivial issues like child app not showing or parent app showing instead of child app....