I wouldn't. but then I'm also shocked at the sorts of things people put on at kid's b'day parties. so the food's about on par really. Exposure to teh brand would definitely be my bigggest concern.
if you don't really know them,then it's easy to make an excuse and not go if you're not comfortable with it. Turning down on invite shouldn't rule you out for all other parties - although if they're all at maccas you might have an issue