I'd do the pizzas. But those little pigs in a blanket are like crack.

Pizza! You can't go wrong for 6 year olds.
It's affordable, easy, and they will probably cut into smaller slices if you ask.
I have done several kid (and grown up) parties with Costco Pizza.
Have fun

maybe their chicken salad too. that stuff is really good.

I like their sliced cheese platters; lots of good dips (that greek yogurt jalapeno dip is amazing) with pita chips; sandwiches or wrap/rolls are great.

I like their frozen apetizers but think that could be a lot of work in terms of heating and getting them out for a party.

Meatballs in a crock pot

The spanakopita is subpar, but the frozen quiches are good. The frozen fruit salad is pretty nice too. You just need to thaw it and pick an easy dressing recipe.

They have lots of frozen kid friendly food -- mini wrapped hot dogs, mini quiches for the adults, wontons, etc. Get a fresh fruit bowl, vege platter; I also bought a box of clementines to put out in a nice bowl which many people thought was a great idea.

the turkey wraps are good. I like the asian chicken better but they are huge. The turkey roll ups are nice and small and great for parties.

Any favorite platters?

I would get everything from Costco. Order cake, get food platters, beverages, napkins, etc.

Sandwich trays
Fruit tray
Pizza
Cake

DD's 6th birthday party is coming up. I usually cook but I'm trying to make it easy on myself this year.

What would you choose from Costco to feed lunch to about 18 kids, plus some about half the parents?