We had my son's 1st at Gymboree and it was so easy! They entertain the kids in their mat/play room with a parachute, songs and bubbles. There is a separate room for food. You can bring your own cake, etc. They provide the goody bags and clean up the mess! By far it was the best idea.
For his 2nd bday I had it at home and it was hectic. I too had 12 kids and panicked. I rented a 6 foot long little table w/chairs from Parties to Go and covered with a Thomas the Train plastic tablecloth. For a craft, we did a little put together foam sticker kit for candy corns that we pasted on paper gift bags (I got at the craft store or Target). I think the best thing was the "decorate your own cupcake". Frost the cupcakes but have licorice strings, gummy worms, m&m's, etc. for decor. The kids loved it. I also had a little tunnel and a playhouse/tent with balls in it. Between those and the balloons, they entertained themselves. It is important to have a set time (2 hours) so that if people drop off their child, they come back and if neighbor kids come, they go home. Everyone was out of there in 2 hours... thank goodness!
Good luck-
D'arcy