Currently it is only possible to search for Maniphest tasks with a specified parent or subtask ID by manipulating the URL. I'm not sure if this was done for any particular reason, it seems to blame back to
This is intentional, since "type a list of IDs into a box" is a pretty rough interface and this capability doesn't seem terribly useful in the general case (i.e., I think it would likely add more complexity than utility).
You can reach this query through this dropdown:
This pattern (a URL-only search parameter) is used in a few places where there's a valid UI path to get to a particular query (e.g., tasks in column X) but actually specifying values manually seems unlikely to be common/useful/realistic for less-sophisticated users.