I would agree with others when they say that there is no particular age in which your child may stop believing in the magic of Christmas, but I still did when 9 years old...
And also there are many things, many activities, many crafts, many places to visit locally, that can all be done on a shoestring budget.
I don't know what you normally would do for Christmas or how big you normally would go, but why not bring back some of the traditions of Christmas past?
Like, having a big long string and stringing a popcorn garland. Personally, I find it to be a frustrating craft because of how often the popcorn would break, but it's extremely cheap, takes time, maybe if you had some hot chocolate and Christmas music going in the background, and at the end, you could hang the string of popcorn outside for the birds...
Or make colored paper chains, you don't even have to go out and buy books of red and green construction paper, just look for stuff you can use around the house, cut it into strips, use a piece of tape or a stapler and make loops one into the next into the next and make a chain of papers that you can hang up. Very inexpensive, and it's fun to decorate this way, and it can be part of the magic of Christmas which is to decorate your home.. to celebrate, to be joyful, to add a little extra color and sparkle into an otherwise dismal part of the year.
To me, crafts that you make yourself and experiences that you do with a loved one or a family are way more important than any experience or item that you can buy. Try researching wartime Christmas traditions. Not a lot of people had a lot of extra money or extra rations or extra materials that were even available in the stores as so much was diverted to the war effort, and yet those families still managed to have a Merry Christmas even under constant mortal threat.
I'm so sorry to hear that you lost your job, really really sorry to hear that. And that you're struggling financially and of course you're going to feel it at Christmas, but this is a great opportunity to show your child without having to explain everything about the grown up world and economics, that Christmas is more than what you can buy in a store.
Hope this helps