I think an ornament would be an excellent idea. However, if you decide to go with holiday cookies (or Gingerbread people), it's really not too difficult for that age.
For several years straight, starting in pre-school, I brought all the supplies to my child's class for either Christmas or valentine cookies. For Christmas, their party usually consisted of refreshments, plus 4 or 5 crafts. Each table held about 6 kids with an adult volunteer or two. The kids rotated tables. Or if cookies were the only activity, they spread out into table groups.
The secret is disposable everything! For cookies, I'd bake enough shaped sugar cookies for each child to do 5 or 6. Plus I'd bring a HUGE bowl of white icing. We used disposable plastic table cloths and then a piece of fresh copy paper as a placemat. (Easy to replace when it got too messy.) I'd bring gallon-size plastic bags and styrofoam plates that were dinner sized, but would slip inside the plastic bag. (Very important to test). Mark the child's name ON TOP of the plate (on the edge) and put aside in a finishing area. As they complete their cookies, they go onto their own plate to dry. At the end of the day, theses are slipped plate and all into the bags for easier transport. (Tell the kids to still hold the plate flat, but at least their car will be protected, and if they do tip the plate, the cookies don't end up on the ground.)
I had all kinds of sugar glitter and sprinkles. We used dixie cups on each table to hold M&M's, chocolate chips, and other decorator bits. We used disposable plastic bowls at each table to hold a reasonable portion of icing. The adult volunteer would help the kids mix colors, so there would be 3 or 4 bowls which they would share and pass around. Plastic knives to spread the frosting.
If the classroom doesn't have a sink in it, bring a big bucket to be filled with soapy dishwater for quick hand cleaning.
At the end, the tables will be a mess. But just pick out any decoratioins you want to save and then bundle up the the plastic cloths and trow it ALL away.
By the way - I bought a lot of the supplies, including sugar glitter, at the dollar store.
Good luck!