I guess I have several thoughts on this one. First, is there a way to get a credit for future flights? Second, I agree that BC is expensive and THAT is a huge expense. Third, if you are still in dire financial times, you need to save money. You don't want your children to have the memory of losing their home because the parents spent money on things other than the necessities first.
Now, I think borrowing money from parents is fine in certain situations. Since you borrowed from her, I am guessing you had no options at a bank. That is either from poor credit or fully extended credit-wise. An expensive trip to WDW is not going to help with your financial sitiuation.
We never went to WDW as a child. Instead, we did go camping every year and went to stay with my grandparents at the beach. These trips did not cost my parents much. Camping was cheap and we drove to Florida and had a kitchen. We did live in a nice house and went to private school. Did we feel like our parents jipped us on vacations? Of course not. I remember making sand castles and fishing.
I am now a single mom putting myself through school. The idea of having substantial debt over my head and spending that much money on WDW is scary to me. And there is no way that your mother will not find out. Your child(ren) will probably want to talk about it. You can't really tell your child(ren) to not tell their grandma about their great trip. And I know I have had to ask my parents for help along the way. I have always paid them back and also offered help with various projects and things as interest. (They wouldn't hear of interest in monetary form)
I just wonder how you could afford laying out money for plane tickets for several people.

I have saved and saved to take my dd to WDW for Christmas. I won't have to worry about owing anyone but for my car note and rent. We are trying to do a lot on a budget so we are staying in one of the nicer DTD hotels at a great value season (going the week before Christmas to get the amazing rates

) and I did rewards programs to pay for two of our five nights.
The point? Don't harm a relationship with your child's grandmother for this. It isn't worth it. If you are financially stable enough to take a high dollar vacation, then get a loan at a bank and pay the woman back.