To be honest, Macca's food is no worse than some of the rubbish I have seen dished up at some kids parties, especially the ones at indoor playcentres. So from that aspect, I have no issue Maccas, at least you have some idea what is in the food, rather than wondering what is in that el cheopo sausage roll.
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My first thought too.

We have Maccas/KFC rarely - once every 4-5 months if that. And then it's only for the nuggets and some fries. My kids (3 and 20 months) just aren't too interested in it. I purposely don't buy the kiddie meals with a toy when we do go there.

Probably going to the party won't make your DS want maccas all the time ... and I guess if you go, and he does become enamoured of ol' Ronald McDonald, then it will just be up to you to not go there in future- know what I mean? By the same token, you shouldn't feel obliged to go when you don't know the parents etc. Just say you have other plans.