Hi Yudhi,
I think disabling foreign keys before maintenance will help.
Do you mean to say call the parent table first for maintenance followed by child, and remove all the foreign key first which are pointing to this parent table partition which is going to be dropped by the pg_partman?
As drop/create partition is being called from within pg_partman without our intervention, so where should we put this drop foreign key code? Do you mean having that with another event trigger which will fire before drop?