I sound like an old man when I say that, once upon a time, advent calendars didn't just contain chocolate! Now, these 'Countdown to Christmas' calendars are all the rage and sell in every shop from Woolies to Coles to...everywhere!
Here's a resource that can value add to those calendars if they're all that's available to you.
I've created a document with 25 short sections, one or two sentences each. These can be cut out as little slips of paper that can be folded and inserted inside the doors of advent calendars, in order to make sure the story of the birth of Jesus gets a look in! They tell the story of the nativity and include some of the Church's traditions about Christmas too. They also enable families to have daily conversations about faith.
If you use a more faith-based calendar you can still use these slips to supplement it.
Have a look and see if it would be helpful to you. If you like it, pass on the link to this page so others can use it too!