This may be a bit complicated for your little ones, but the idea may give you a springboard for something similar...
Last year, my DH and his siblings wanted to give DMIL a clothes dryer for Christmas

Anyway... I created a scavenger hunt using the "Twas the Night Before Christmas" poem, changing wording where necessary to give her a clue to direct her to the next clue. It started with unwrapping a gift from under the Christmas tree. Inside the package was the clue:
Twas the night before Christmas, and all through the house
Not a creature was stirring, not even a mouse.
The stockings were hung by the chimney with care
In hopes that Oma would soon look there.
Of course, there was a Christmas stocking hung on the mantle with clue #2 tucked inside it. On through the poem we went, until finally the hunt ended in Oma's laundry room.
The biggest surprise was she had absolutely no idea we had gotten a new dryer into the house two days before Christmas without her knowing.
Have a very Merry Christmas and more fun than you can stand!!