Since they've never had a pet, why not do sort of a personalized Advent Calendar (where you open the little windows and count down to the special day)?
Wrap up boxes, one to be opened each day. And I wouldn't have the last one be on Christmas Day itself, but a couple of days later, so maybe start on the 23rd and end on the 27th or 28th.
Inside the first box, put something that will get them thinking, like a little container of sprinkles like you suggested.
In the second box, to be opened on the second day, put a blank calendar. When they get the pet, they can fill in the calendar with chores like "replace the bedding" or "check the water" and "feed Sprinkles", and they can divide up the chores and be responsible for checking off a completed chore. But they won't guess what a blank calendar is for!
The third box can be a bottle of water and some healthy food, like an apple or some oatmeal. This will relate to the fact that they'll have to keep the pet stocked with proper food and water. Later they'll have to learn what to feed their pet and what not to feed it.
Find out what kind of bedding hamsters require and put a clue to that in a box. If it's pine shavings, put a pine cone in the box.
The next box can be a book about taking care of a pet (any pet). Make sure they read it before they can open the next box.
Then the final box can be a hamster wheel and that can be the day you take them to the pet store.