I didn't start a Christmas Eve box until DD was about 4. But I had bought a copy of "Twas the night before Christmas" for her first Christmas, and we read that on Christmas Eve every year, even still (she's about to turn 11).
Our Christmas Eve box has:
Family stuff that is used every year:
Twas the night before Christmas book
Hot water bottle with a Snowman cover
Christmas plate (for Santa's cookies)
Christmas plastic glass (for Santa's milk)
DD's stocking
My Christmassy slipper socks
New stuff:
PJs for DH, DD and I
Slippers/slipper socks for DD
Lush Bath Bomb each for DD and I
(Sometimes a festive or manly shower gel for DH)
Hot Chocolate (I tend to do the "lump of chocolate on a wooden spoon" luxury ones for Christmas) for all 3 of us
We have dinner, DD (as the youngest) lights the Christmas Candle (Irish tradition) and we have a small quiet time together remembering the good and bad things about the year and people no longer with us for Christmas, then the box comes out for DD to set up for Santa and head up for a bath on her way to bed. Comes back down for hot choc and a spare cookie (we bake in the afternoon for Santa), and then story in bed.
The plate and glass, and sometimes the hot water bottle too, have been used throughout December, but are part of the tradition of the Box. DD knows, and always has known, that it is us who do the Box - whereas Santa leaves everything in the stocking. (So she started a few years ago trying to make sure that the "right" PJs were bought, and looking herself for nice ones to get for DH and pointing those out to me!!
)