Targeting sub fields on a repeater is possible, but a bit more complicated.
You have the half of it here
var $fieldTarget_id = $input.closest('.acf-row').data('id');
What you need to do is use .closest
to get the row and then .find
to find the other field that you want to target on the same row.
var $row = $input.closest('.acf-row');
var $target = $row.find('[data-key="'+ $fieldTarget_key+'"] input.hasDatepicker');