My helper (FDW) is pretty awesome. She's been here for less than a year, but takes good initiatives and a lot of care for my twin babies. We are blessed to have such a good helper. How should I give her a little reward? What you guys think?

How about bringing her on a shopping trip to buy some stuff to send back to her family? Rather than just giving money (though very practical), she can buy something for them (since she probably has been sending the bulk of her pay back). If not, a nice treat as a reward would be good too! It is very difficult to find a good helper and it is really sweet of you to find ways to show your appreciation!