Something very similar happened to me yesterday. DD's school was having a fund raiser. One of the Arby's here donated 20% of all sales that day to the school, and the faculty also chipped in at the store serving food and cleaning tables etc. We HAD to go. I hate Arby's only because of how I feel after eating there. (while eating, I'm in heaven!

) Soooooo.....I just didn't eat there. We let the kids eat then we went to OUTBACK!!!!!! I had salmon (1/3 of what they brought, wrapped up the rest), steamed veggies (Yea right! Steamed in butter and salt!), and a salad and a glass of wine!
Maybe you could grab something healthy before you go, and have a diet soda or something while the kids eat. It is a huge mental boost when you can walk out of any FF joint, and not have eaten there!

Other than that, I have no practical suggestions, since I'm not sure what plan you are following, I'm assuming WW type, since the salad dressing scares you. Maybe a grilled chicken sandwich with mayo on the side and a diet soda?
Whatever you decide, just keep up the program, no matter what happens at Mc D's!
Keep the Faith!
Tracy