I have always leaned towords a slightly shorter board when setting up my begginer friends... This was purely based on my own logic of, begginers wont be pushing the limits of a board and a shorter board will be easier to throw around.
That's fine, but as boarderholic mentioned, (...she's a female, btw. So her advice is directly relevant to your wife's/daughter's situation!)
Based on the proper weight/size range. The 138 will likely be way
too short. Find the proper size for her (...and you daughter) based on weight and then
size up or down from there. I ride 2 short-ish
boards that I'm well over the weight range for, but I learned on a much longer board!
If you start from the inappropriate size, based on wrong methods, the size adjustments you make for beginners, or what you hope she will grow into, start from the wrong size. So any potential problems are compounded by that, and solving any learning, riding, technique issues they might have? Will be that much more complicated as well!
We're not talking down to you, busting your chops or trying to steer you wrong!
Check out Wiredsports "Fit Guides" web page. You will see what we're talking about!