Some suggestions:
1) Give function a returntype.
2)Use a select query to return the result set, not an insert query.
3) It may be inefficient to pass the whole form object, when the function requires just one or two parameters from the form.
4) You imply that form.backup is a list. It is poor design to pass a list as a form field.
5) You loop through the list arguments.form.backup, storing each list element in the variable backup. However, backup occurs nowhere in the loop.
Here follows a guess from me:
<cffunction name="add" access="public" returntype="numeric">
<cfargument name="form" type="struct" required="yes">
<cfset var idList = "">
<!---insert into parent table--->
<cfquery name="insert" datasource="#request.dsn#">
...insert goes here---
</cfquery>
<!--- my guess for select query --->
<cfquery name="getID" datasource="#request.dsn#" result="returnID">
SELECT identityCol
FROM myTBL
WHERE vou in (/* list goes here*/)
</cfquery>
<cfset idList = valueList(returnID.identityCol)>
<!--- loop through IDs, inserting into each child table--->
<cfloop index="id" list="#idList#">
<cfquery name="insert_backup" datasource="#request.dsn#">
INSERT into backups (vou, type_id)
VALUES
(
<cfqueryparam value="#arguments.form.vou#" cfsqltype="cf_sql_varchar" maxlength="255" />,
<cfqueryparam value="#id#" cfsqltype="cf_sql_integer" />
)
</cfquery>
</cfloop>
<!---return last ID --->
<cfreturn listLast(idList)>
</cffunction>
