Chained slects placeholders - how to modify

We have dynamically populated a chained select using the gform_chained_selects_input_choices filter, all works fine. But having trouble in changing the default ‘children’ and ‘parents’ placeholders.
So for the first dropdown we use gform_chained_selects_input_choices_3_1_1

I then want to use the pre_render filter to set the placeholder text.
So we would loop through the fields and set the placeholder like this:
$field->placeholder = ‘Pick an area’; for the first dropdown.
But how do you do this for a chained dropdown?
Many thanks for any help

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