when is a DELETE FK trigger planned?

I have a cascading delete trigger that is obviously using a seqscan.
(Explain analyze shows that trigger as taking over 1000s while all
other triggers are <1s. The value in test delete didn't even appear in
this child table, so an index scan would have been almost instant.)

If I do
DELETE FROM child_table WHERE fkey = value;
I get an index scan. Why doesn't the trigger do that, and how can I
force it to re-plan?