Take those kids away for the weekend! And get D13 involved in planning a vacation for spring break. I know money is tight, but honestly, for $1500 on your credit card you could take them somewhere for a nice break. I do NOT approve of credit card debt but if ever there was a time for it, this is it.
Start looking into thrifty vacations that they could both enjoy.
As hard as it is for you to have this in your face, it's gotta be even harder for D13 who is probably hearing about it from her classmates.