If you want to remove date of birth field which is available in traveler form in checkout page, please add following given code in functions.php file of your child theme.

function wp_travel_remove_dob( $checkout_fields ){

	unset( $checkout_fields['traveller_fields']['dob'] );
	return $checkout_fields;

add_filter('wp_travel_checkout_fields', 'wp_travel_remove_dob' );


After Removing Field:

Also, regarding creating child theme, please refer link.

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