Simply have your Function return the value from the Recordset:
Public Function rubrieknaamSQL(Child As Integer)
Dim rst As DAO.Recordset
Dim strSQL As String
strSQL = "SELECT rubrieknaam FROM dbo_tbl_rubriek where rubrieknummer = " & Child & ""
Set rst = CurrentDb.OpenRecordset(strSQL)
' new code:
rubrieknaamSQL = rst!rubrieknaam
rst.Close
Set rst = Nothing
End Function