Oh e-hell no. If Auntie wants to go Auntie can pay for it. If it was a financial issue, I would offer to pay. Otherwise, no way.
When I was a nanny and college student, and even later as a nanny and married, I often went to my "kids" performances. It NEVER occurred to me to ask the families to pay. If the performance was too expensive (like some ballet performances), I just didn't go. For the record, I also would pay from my own funds to take the kids to museums, ice cream, lunch, etc. Sometimes the parents would pay me back, but I never assumed or asked. I also attended classroom shows, classroom birthday parties, VIP days, Friday afternoon Shabbot sing-alongs - all on my own time and own dime. I did it out of love. Because I loved THEM. It was a favor, a gift, to ME to be able to be there and experience those things with them. I wasn't doing them a favor by showing up.
Auntie needs a kick in the pants. If you wants to go to the performances, then pay up. Or stay home. No makes her spoil your kids. She could stop spending so much on gifts and save up for tickets if she wants to see the shows. I would continue telling her about the events along with the cost, and add, "and if you can't make it, it's ok." Then it's up to her. She's not obligated and you aren't paying.