So thanks to all of the good advice here, I just had a fairly good conversation with my mom. I especially tried incorporating the advice of those who told me not to think of it as a confrontation, but just a matter of fact discussion. I did everything I could to keep it extremely rational, and presented the situation in as cool and factual a manner possible, rather than like I was anticipating an argument. I told her that there was a large chunk of money left in the account because we had not needed it. She was surprised and not happy, saying "that wasn't what we wanted." Rather than getting into what they wanted, or trying to rehash our reasons for wanting to pay for it ourselves, I just explained that we had saved enough to pay for things, and that regardless of anyone's wishes, the fact was that the money was there and we needed to decide what to do with it.
I really appreciate all of the input. Some of you mustachians are extremely perceptive folks. It was a delicate situation (probably still is - it's not completely resolved yet), but I think I handled it really well. I presented a few ideas for what could be done with the money, but for me, the hardest part - telling her we didn't use it - is behind me.