I was a room parent for many years in my daughter's classrooms and others who didn't have anyone and want to point a few things out (not trying to be a downer ; ) I can honestly say that 1 hour isn't long at all, so plan for a game OR a craft, not both. Before you plan find out from the teacher what she has planned, she may have something she's going to do at that time with the students, in my experience many teachers did, so there goes a little portion of your time. And, most teachers have the room decorated already for the month of December so go along with that theme. You may want to just have small decorations for each child's desk or table that they can take home, little sugar ice cream cone "trees" are cute, fun and can be part of their treats.
To make: Spread a cone with green icing; spoon a dollop of white icing onto a sugar cookie, then gently push the inverted cone into the icing. Top the icing with shredded coconut for snow. Decorate the tree with sprinkles and candy for ornaments, (attach with a tiny bit of icing) and crown with a lollipop star (work a hole in the cone for the stem) or simply a Starlight Mint attached with icing. Provide brown bags so the children can take home their tree or whatever pieces remain of it.
Large snowflakes out of paper that the children fold and make cuts and open out are fun, you could let them go over them with a glue stick (doesn't need to cover the whole thing or be perfect) and have a parent sprinkle glitter over it (over a bowl or large piece of paper to catch the excess glitter.) Make a hole in the top of it and tie a piece of yarn through it so they can hang it at home. Another fun, easy craft is bookmarks they make out of craft foam cut-outs (I buy them in buckets at Michael's) and glue to craft sticks.
At this age treats should be simple for them to eat out of hand like Rice Krispie treats (I put them in molds to suit the holiday), sugar cookies (if they are big enough you can write each child's name on theirs), gingerbread boys and girls or holiday colored doughnuts. Cupcakes (a fave of mine) may be too messy for them to eat and involve extra clean-up time. Drinks should be something other than soda, apple cider is a great alternative for a holiday party. If you have candies for the children serve that with their treats and drinks. Have some cute cello bags to put goodies in for a student to take home, it seemed there was always one or two who wouldn't eat the goodies, because they weren't allowed to eat anything not provided by mom or dad, they didn't like sweets or the ones we had, or didn't celebrate the holiday. I always respected their reason and helped them pack their goodies to take, IF they wanted them.
Hope you have lots of fun!!