Hello Ian, and welcome to the Disney Parks Moms Panel!
You're in for such a treat at Mickey's Very Merry Christmas Party! I love the shimmery layer of ice lights on Cinderella Castle, Christmas cookies and cocoa, a special parade - and snow! Ok, not the kind you're used to in Canada, but this Florida girl will take it however I can get it.
I just called Walt Disney World Guest Relations for an answer to your question. The Cast Member I spoke with said that to pick up the physical tickets to the party at Will-Call, you would need the original credit card used to place the order, along with a photo I.D. Since your mother bought your tickets with her card, unfortunately you won't be able to pick them up at the event. You can elect to have the tickets mailed to you at home - since you're planning a holiday trip, you still have plenty of time to receive them before your visit. Also, because your party tickets are linked to your My Disney Experience account, you can use MagicBands for entry, if you have them.
I hope this helps with your planning, Ian. Please stop by and see us again if you have any more questions. Wishing you a jolly Disney holiday!
Keep Disney Dreaming,
Kathy K.