Go to the lunch.

Have you called the restaurant to see if they can do something healthier for you?

If you attend, you might want to consider doing something similar to what TrojanHorse suggested. Eat prior to attending the lunch. I'm not suggesting you don't eat at lunch, because I think when you're out with kids, it's a little easier to not eat. I think here, you run the risk of offending someone if you don't eat something. However, eat something similar, but a bit smaller, than what you'd normally eat for lunch, before you go to the department lunch. At the department lunch, eat the veggie and hummus starter plate as your lunch. Drink lots of water, and if you still want dessert after you eat the starter plate, ask for fresh fruit as dessert.