Just tell them, "hey babe, I've got something for you, but this is an online money and you can only cash it out by exchanging it. And after we converted to real money, we now have our date and buy anything what you want."
First you need to tell her how to earn it, spending is very easy and doesn't require any efforts but to earn it requires great level of dedication.
But in the case of bitcoin , earning it is much more easier than spending it, as I have saw a lot of ways of earning bitcoin online but for spending there are a limited stores (but still sufficient) .Actually I want big companies to adopt bitcoin.